email-validator: Validate Email Addresses

A robust email address syntax and deliverability validation library for Python 3.8+ by Joshua Tauberer.

This library validates that a string is of the form name@example.com and optionally checks that the domain name is set up to receive email. This is the sort of validation you would want when you are identifying users by their email address like on a registration/login form (but not necessarily for composing an email message, see below).

Key features:

• Checks that an email address has the correct syntax --- good for registration/login forms or other uses related to identifying users.
• Gives friendly English error messages when validation fails that you can display to end-users.
• Checks deliverability (optional): Does the domain name resolve? (You can override the default DNS resolver to add query caching.)
• Supports internationalized domain names and internationalized local parts.
• Rejects addresses with unsafe Unicode characters, obsolete email address syntax that you'd find unexpected, special use domain names like @localhost, and domains without a dot by default. This is an opinionated library!
• Normalizes email addresses (important for internationalized and quoted-string addresses! see below).
• Python type annotations are used.

This is an opinionated library. You should definitely also consider using the less-opinionated pyIsEmail and flanker if they are better for your use case.

View the CHANGELOG / Release Notes for the version history of changes in the library. Occasionally this README is ahead of the latest published package --- see the CHANGELOG for details.

Installation

This package is on PyPI, so:

pip install email-validator

(You might need to use pip3 depending on your local environment.)

Quick Start

If you're validating a user's email address before creating a user account in your application, you might do this:

from email_validator import validate_email, EmailNotValidError

email = "my+address@example.org"

try:

  # Check that the email address is valid. Turn on check_deliverability
  # for first-time validations like on account creation pages (but not
  # login pages).
  emailinfo = validate_email(email, check_deliverability=False)

  # After this point, use only the normalized form of the email address,
  # especially before going to a database query.
  email = emailinfo.normalized

except EmailNotValidError as e:

  # The exception message is human-readable explanation of why it's
  # not a valid (or deliverable) email address.
  print(str(e))

This validates the address and gives you its normalized form. You should put the normalized form in your database and always normalize before checking if an address is in your database. When using this in a login form, set check_deliverability to False to avoid unnecessary DNS queries.

Usage

Overview

The module provides a function validate_email(email_address) which takes an email address and:

• Raises a EmailNotValidError with a helpful, human-readable error message explaining why the email address is not valid, or
• Returns an object with a normalized form of the email address (which you should use!) and other information about it.

When an email address is not valid, validate_email raises either an EmailSyntaxError if the form of the address is invalid or an EmailUndeliverableError if the domain name fails DNS checks. Both exception classes are subclasses of EmailNotValidError, which in turn is a subclass of ValueError.

But when an email address is valid, an object is returned containing a normalized form of the email address (which you should use!) and other information.

The validator doesn't, by default, permit obsoleted forms of email addresses that no one uses anymore even though they are still valid and deliverable, since they will probably give you grief if you're using email for login. (See later in the document about how to allow some obsolete forms.)

The validator optionally checks that the domain name in the email address has a DNS MX record indicating that it can receive email. (Except a Null MX record. If there is no MX record, a fallback A/AAAA-record is permitted, unless a reject-all SPF record is present.) DNS is slow and sometimes unavailable or unreliable, so consider whether these checks are useful for your use case and turn them off if they aren't. There is nothing to be gained by trying to actually contact an SMTP server, so that's not done here. For privacy, security, and practicality reasons, servers are good at not giving away whether an address is deliverable or not: email addresses that appear to accept mail at first can bounce mail after a delay, and bounced mail may indicate a temporary failure of a good email address (sometimes an intentional failure, like greylisting).

Options

The validate_email function also accepts the following keyword arguments (defaults are as shown below):

check_deliverability=True: If true, DNS queries are made to check that the domain name in the email address (the part after the @-sign) can receive mail, as described above. Set to False to skip this DNS-based check. It is recommended to pass False when performing validation for login pages (but not account creation pages) since re-validation of a previously validated domain in your database by querying DNS at every login is probably undesirable. You can also set email_validator.CHECK_DELIVERABILITY to False to turn this off for all calls by default.

dns_resolver=None: Pass an instance of dns.resolver.Resolver to control the DNS resolver including setting a timeout and a cache. The caching_resolver function shown below is a helper function to construct a dns.resolver.Resolver with a LRUCache. Reuse the same resolver instance across calls to validate_email to make use of the cache.

test_environment=False: If True, DNS-based deliverability checks are disabled and test and **.test domain names are permitted (see below). You can also set email_validator.TEST_ENVIRONMENT to True to turn it on for all calls by default.

allow_smtputf8=True: Set to False to prohibit internationalized addresses that would require the SMTPUTF8 extension. You can also set email_validator.ALLOW_SMTPUTF8 to False to turn it off for all calls by default.

allow_quoted_local=False: Set to True to allow obscure and potentially problematic email addresses in which the part of the address before the @-sign contains spaces, @-signs, or other surprising characters when the local part is surrounded in quotes (so-called quoted-string local parts). In the object returned by validate_email, the normalized local part removes any unnecessary backslash-escaping and even removes the surrounding quotes if the address would be valid without them. You can also set email_validator.ALLOW_QUOTED_LOCAL to True to turn this on for all calls by default.

allow_domain_literal=False: Set to True to allow bracketed IPv4 and "IPv6:"-prefixd IPv6 addresses in the domain part of the email address. No deliverability checks are performed for these addresses. In the object returned by validate_email, the normalized domain will use the condensed IPv6 format, if applicable. The object's domain_address attribute will hold the parsed ipaddress.IPv4Address or ipaddress.IPv6Address object if applicable. You can also set email_validator.ALLOW_DOMAIN_LITERAL to True to turn this on for all calls by default.

allow_empty_local=False: Set to True to allow an empty local part (i.e. @example.com), e.g. for validating Postfix aliases.

DNS timeout and cache

When validating many email addresses or to control the timeout (the default is 15 seconds), create a caching dns.resolver.Resolver to reuse in each call. The caching_resolver function returns one easily for you:

from email_validator import validate_email, caching_resolver

resolver = caching_resolver(timeout=10)

while True:
  validate_email(email, dns_resolver=resolver)

Test addresses

This library rejects email addresses that use the Special Use Domain Names invalid, localhost, test, and some others by raising EmailSyntaxError. This is to protect your system from abuse: You probably don't want a user to be able to cause an email to be sent to localhost (although they might be able to still do so via a malicious MX record). However, in your non-production test environments you may want to use @test or @myname.test email addresses. There are three ways you can allow this:

1. Add test_environment=True to the call to validate_email (see above).
2. Set email_validator.TEST_ENVIRONMENT to True globally.
3. Remove the special-use domain name that you want to use from email_validator.SPECIAL_USE_DOMAIN_NAMES, e.g.:

import email_validator
email_validator.SPECIAL_USE_DOMAIN_NAMES.remove("test")

It is tempting to use @example.com/net/org in tests. They are not in this library's SPECIAL_USE_DOMAIN_NAMES list so you can, but shouldn't, use them. These domains are reserved to IANA for use in documentation so there is no risk of accidentally emailing someone at those domains. But beware that this library will nevertheless reject these domain names if DNS-based deliverability checks are not disabled because these domains do not resolve to domains that accept email. In tests, consider using your own domain name or @test or @myname.test instead.

Internationalized email addresses

The email protocol SMTP and the domain name system DNS have historically only allowed English (ASCII) characters in email addresses and domain names, respectively. Each has adapted to internationalization in a separate way, creating two separate aspects to email address internationalization.

Internationalized domain names (IDN)

The first is internationalized domain names (RFC 5891), a.k.a IDNA 2008. The DNS system has not been updated with Unicode support. Instead, internationalized domain names are converted into a special IDNA ASCII "Punycode" form starting with xn--. When an email address has non-ASCII characters in its domain part, the domain part is replaced with its IDNA ASCII equivalent form in the process of mail transmission. Your mail submission library probably does this for you transparently. (Compliance around the web is not very good though.) This library conforms to IDNA 2008 using the idna module by Kim Davies.

Internationalized local parts

The second sort of internationalization is internationalization in the local part of the address (before the @-sign). In non-internationalized email addresses, only English letters, numbers, and some punctuation (._!#$%&'^``*+-=~/?{|}) are allowed. In internationalized email address local parts, a wider range of Unicode characters are allowed.

A surprisingly large number of Unicode characters are not safe to display, especially when the email address is concatenated with other text, so this library tries to protect you by not permitting resvered, non-, private use, formatting (which can be used to alter the display order of characters), whitespace, and control characters, and combining characters as the first character of the local part and the domain name (so that they cannot combine with something outside of the email address string or with the @-sign). See https://qntm.org/safe and https://trojansource.codes/ for relevant prior work. (Other than whitespace, these are checks that you should be applying to nearly all user inputs in a security-sensitive context.)

These character checks are performed after Unicode normalization (see below), so you are only fully protected if you replace all user-provided email addresses with the normalized email address string returned by this library. This does not guard against the well known problem that many Unicode characters look alike (or are identical), which can be used to fool humans reading displayed text.

Email addresses with these non-ASCII characters require that your mail submission library and the mail servers along the route to the destination, including your own outbound mail server, all support the SMTPUTF8 (RFC 6531) extension. Support for SMTPUTF8 varies. See the allow_smtputf8 parameter.

If you know ahead of time that SMTPUTF8 is not supported by your mail submission stack

By default all internationalized forms are accepted by the validator. But if you know ahead of time that SMTPUTF8 is not supported by your mail submission stack, then you must filter out addresses that require SMTPUTF8 using the allow_smtputf8=False keyword argument (see above). This will cause the validation function to raise a EmailSyntaxError if delivery would require SMTPUTF8. That's just in those cases where non-ASCII characters appear before the @-sign. If you do not set allow_smtputf8=False, you can also check the value of the smtputf8 field in the returned object.

If your mail submission library doesn't support Unicode at all --- even in the domain part of the address --- then immediately prior to mail submission you must replace the email address with its ASCII-ized form. This library gives you back the ASCII-ized form in the ascii_email field in the returned object, which you can get like this:

emailinfo = validate_email(email, allow_smtputf8=False)
email = emailinfo.ascii_email

The local part is left alone (if it has internationalized characters allow_smtputf8=False will force validation to fail) and the domain part is converted to IDNA ASCII. (You probably should not do this at account creation time so you don't change the user's login information without telling them.)

Normalization

Unicode Normalization

The use of Unicode in email addresses introduced a normalization problem. Different Unicode strings can look identical and have the same semantic meaning to the user. The normalized field returned on successful validation provides the correctly normalized form of the given email address.

For example, the CJK fullwidth Latin letters are considered semantically equivalent in domain names to their ASCII counterparts. This library normalizes them to their ASCII counterparts:

emailinfo = validate_email("me@Ｄｏｍａｉｎ.com")
print(emailinfo.normalized)
print(emailinfo.ascii_email)
# prints "me@domain.com" twice

Because an end-user might type their email address in different (but equivalent) un-normalized forms at different times, you ought to replace what they enter with the normalized form immediately prior to going into your database (during account creation), querying your database (during login), or sending outbound mail. Normalization may also change the length of an email address, and this may affect whether it is valid and acceptable by your SMTP provider.

The normalizations include lowercasing the domain part of the email address (domain names are case-insensitive), Unicode "NFC" normalization of the whole address (which turns characters plus combining characters into precomposed characters where possible, replacement of fullwidth and halfwidth characters in the domain part, possibly other UTS46 mappings on the domain part, and conversion from Punycode to Unicode characters.

(See RFC 6532 (internationalized email) section 3.1 and RFC 5895 (IDNA 2008) section 2.)

Other Normalization

Normalization is also applied to quoted-string local parts and domain literal IPv6 addresses if you have allowed them by the allow_quoted_local and allow_domain_literal options. In quoted-string local parts, unnecessary backslash escaping is removed and even the surrounding quotes are removed if they are unnecessary. For IPv6 domain literals, the IPv6 address is normalized to condensed form. RFC 2142 also requires lowercase normalization for some specific mailbox names like postmaster@.

Length checks

This library checks that the length of the email address is not longer than the maximum length. The check is performed on the normalized form of the address, which might be different from a string provided by a user. If you send email to the original string and not the normalized address, the email might be rejected because the original address could be too long.

Examples

For the email address test@joshdata.me, the returned object is:

ValidatedEmail(
  normalized='test@joshdata.me',
  local_part='test',
  domain='joshdata.me',
  ascii_email='test@joshdata.me',
  ascii_local_part='test',
  ascii_domain='joshdata.me',
  smtputf8=False)

For the fictitious but valid address example@ツ.ⓁⒾⒻⒺ, which has an internationalized domain but ASCII local part, the returned object is:

ValidatedEmail(
  normalized='example@ツ.life',
  local_part='example',
  domain='ツ.life',
  ascii_email='example@xn--bdk.life',
  ascii_local_part='example',
  ascii_domain='xn--bdk.life',
  smtputf8=False)

Note that normalized and other fields provide a normalized form of the email address, domain name, and (in other cases) local part (see earlier discussion of normalization), which you should use in your database.

Calling validate_email with the ASCII form of the above email address, example@xn--bdk.life, returns the exact same information (i.e., the normalized field always will contain Unicode characters, not Punycode).

For the fictitious address ツ-test@joshdata.me, which has an internationalized local part, the returned object is:

ValidatedEmail(
  normalized='ツ-test@joshdata.me',
  local_part='ツ-test',
  domain='joshdata.me',
  ascii_email=None,
  ascii_local_part=None,
  ascii_domain='joshdata.me',
  smtputf8=True)

Now smtputf8 is True and ascii_email is None because the local part of the address is internationalized. The local_part and normalized fields return the normalized form of the address.

Return value

When an email address passes validation, the fields in the returned object are:

Assumptions

By design, this validator does not pass all email addresses that strictly conform to the standards. Many email address forms are obsolete or likely to cause trouble:

• The validator assumes the email address is intended to be usable on the public Internet. The domain part of the email address must be a resolvable domain name (see the deliverability checks described above). Most Special Use Domain Names and their subdomains, as well as domain names without a ., are rejected as a syntax error (except see the test_environment parameter above).
• Obsolete email syntaxes are rejected: The unusual "(comment)" syntax is rejected. Extremely old obsolete syntaxes are rejected. Quoted-string local parts and domain-literal addresses are rejected by default, but there are options to allow them (see above). No one uses these forms anymore, and I can't think of any reason why anyone using this library would need to accept them.

Testing

Tests can be run using

pip install -r test_requirements.txt 
make test

Tests run with mocked DNS responses. When adding or changing tests, temporarily turn on the BUILD_MOCKED_DNS_RESPONSE_DATA flag in tests/mocked_dns_responses.py to re-build the database of mocked responses from live queries.

For Project Maintainers

The package is distributed as a universal wheel and as a source package.

To release:

• Update CHANGELOG.md.
• Update the version number in email_validator/version.py.
• Make & push a commit with the new version number and make sure tests pass.
• Make & push a tag (see command below).
• Make a release at https://github.com/JoshData/python-email-validator/releases/new.
• Publish a source and wheel distribution to pypi (see command below).

git tag v$(cat email_validator/version.py  | sed "s/.* = //" | sed 's/"//g')
git push --tags
./release_to_pypi.sh