email_validator

A robust email address syntax and deliverability validation library for Python 2.7/3.4 by Joshua Tauberer.

This library validates that address are of the form [email protected]. This is the sort of validation you would want for a login form on a website.

Key features:

• Good for validating email addresses used for logins/identity.
• Friendly error messages when validation fails (appropriate to show to end users).
• (optionally) Checks deliverability: Does the domain name resolve?
• Supports internationalized domain names and (optionally) internationalized local parts.
• Normalizes email addresses (super important for internationalized addresses! see below).

The library is NOT for validation of the To: line in an email message (e.g. My Name <[email protected]>), which flanker is more appropriate for. And this library does NOT permit obsolete forms of email addresses, so if you need strict validation against the email specs exactly, use pyIsEmail.

The current version is 1.0.3 (Sept 12, 2017). The only changes since 1.0.0 (Sept 5, 2015) have been small bug and packaging fixes.

Installation

This package is on PyPI, so:

pip install email_validator

pip3 also works.

Usage

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

from email_validator import validate_email, EmailNotValidError

email = "[email protected]"

try:
    v = validate_email(email) # validate and get info
    email = v["email"] # replace with normalized form
except EmailNotValidError as e:
    # email is not valid, exception message is human-readable
    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.

The validator will accept internationalized email addresses, but email addresses with non-ASCII characters in the local part of the address (before the @-sign) require the SMTPUTF8 extension which may not be supported by your mail submission library or your outbound mail server. If you know ahead of time that SMTPUTF8 is not supported then add the keyword argument allow_smtputf8=False to fail validation for addresses that would require SMTPUTF8:

validate_email(email, allow_smtputf8=False)

Overview

The module provides a function validate_email(email_address) which takes an email address (either a str or ASCII bytes) and:

• Raises a EmailNotValidError with a helpful, human-readable error message explaining why the email address is not valid, or
• Returns a dict with information about the deliverability of the email address.

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 does not resolve. Both exception classes are subclasses of EmailNotValidError, which in turn is a subclass of ValueError.

But when an email address is valid, a dict is returned containing information that might aid deliverability (see below).

The validator doesn't 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 that.)

The validator checks that the domain name in the email address resolves. 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).

The function also accepts the following keyword arguments (default as shown):

allow_smtputf8=True

Set to False to prohibit internationalized addresses that would require the SMTPUTF8 extension.

check_deliverability=True

Set to False to skip the domain name resolution check.

allow_empty_local=False

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

Internationalized email addresses

The email protocol SMTP and the domain name system DNS have historically only allowed 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 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. Note that most web browsers are currently in transition between IDNA 2003 (RFC 3490) and IDNA 2008 (RFC 5891) and compliance around the web is not very good in any case, so be aware that edge cases are handled differently by different applications and libraries. 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). These email addresses 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.

How this module works

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 dict.

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 email_ascii field in the returned dict, which you can get like this:

v = validate_email(email, allow_smtputf8=False)
email = v['email_ascii']

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.)

UCS-4 support required for Python 2.7

Note that when using Python 2.7, it is required that it was built with UCS-4 support (see here); otherwise emails with unicode characters outside of the BMP (Basic Multilingual Plane) will not validate correctly.

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 email field returned on successful validation provides the correctly normalized form of the given email address:

v = validate_email(email)
email = v['email']

Because you may get an email address in a variety of forms, you ought to replace it with its normalized form immediately prior to going into your database (during account creation), querying your database (during login), or sending outbound mail.

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 and replaces certain Unicode characters (such as angstrom and ohm) with other equivalent code points (a-with-ring and omega, respectively)), replacement of fullwidth and halfwidth characters in the domain part, and possibly other UTS46 mappings on the domain part.

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

Examples

For the email address [email protected], the returned dict is:

{
  "email": "[email protected]",
  "email_ascii": "[email protected]",
  "local": "test",
  "domain": "example.org",
  "domain_i18n": "example.org",

  "smtputf8": false,

  "mx": [
    [
      0,
      "93.184.216.34"
    ]
  ],
  "mx-fallback": "A"
}

For the fictitious address example@良好Mail.中国, which has an internationalized domain but ASCII local part, the returned dict is:

{
  "email": "example@良好mail.中国",
  "email_ascii": "[email protected]",
  "local": "example",
  "domain": "xn--mail-p86gl01s.xn--fiqs8s",
  "domain_i18n": "良好mail.中国",

  "smtputf8": false,

  "mx": [
    [
      0,
      "218.241.116.40"
    ]
  ],
  "mx-fallback": "A"
}

Note that smtputf8 is False even though the domain part is internationalized because SMTPUTF8 is only needed if the local part of the address is internationalized (the domain part can be converted to IDNA ASCII). Also note that the email and domain_i18n fields provide a normalized form of the email address and domain name (casefolding and Unicode normalization as required by IDNA 2008).

For the fictitious address 树大@occams.info, which has an internationalized local part, the returned dict is:

{
  "email": "树大@occams.info",
  "local": "树大",
  "domain": "occams.info",
  "domain_i18n": "occams.info",

  "smtputf8": true,

  "mx": [
    [
      10,
      "box.occams.info"
    ]
  ],
  "mx-fallback": false
}

Now smtputf8 is True and email_ascii is missing because the local part of the address is internationalized. The local and email fields return the normalized form of the address: certain Unicode characters (such as angstrom and ohm) may be replaced by other equivalent code points (a-with-ring and omega).

Return value

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

email

The canonical form of the email address, mostly useful for display purposes. This merely combines the local and domain_i18n fields (see below).

email_ascii

If present, an ASCII-only form of the email address by replacing the domain part with IDNA ASCII. This field will be present when an ASCII-only form of the email address exists (including if the email address is already ASCII). If the local part of the email address contains internationalized characters, email_ascii will not be present.

local

The local part of the given email address (before the @-sign) with Unicode NFC normalization applied.

domain

The IDNA ASCII-encoded form of the domain part of the given email address (after the @-sign), as it would be transmitted on the wire.

domain_i18n

The canonical internationalized form of the domain part of the address, by round-tripping through IDNA ASCII. If the returned string contains non-ASCII characters, either the SMTPUTF8 feature of MTAs will be required to transmit the message or else the email address('s domain part) must be converted to IDNA ASCII first (given in the returned domain field).

smtputf8

A boolean indicating that the SMTPUTF8 feature of MTAs will be required to transmit messages to this address because the local part of the address has non-ASCII characters (the local part cannot be IDNA-encoded). If allow_smtputf8=False is passed as an argument, this flag will always be false because an exception is raised if it would have been true.

mx

A list of (priority, domain) tuples of MX records specified in the DNS for the domain (see RFC 5321 section 5).

mx-fallback

None if an MX record is found. If no MX records are actually specified in DNS and instead are inferred, through an obsolete mechanism, from A or AAAA records, the value is the type of DNS record used instead (A or AAAA).

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 deliverable on the public Internet using DNS, and so the domain part of the email address must be a resolvable domain name.
• The "quoted string" form of the local part of the email address (RFC 5321 4.1.2) is not permitted --- no one uses this anymore anyway. Quoted forms allow multiple @-signs, space characters, and other troublesome conditions.
• The "literal" form for the domain part of an email address (an IP address) is not accepted --- no one uses this anymore anyway.

Testing

Tests can be run using

make test

For Project Maintainers

The package is distributed as a universal wheel. The wheel is specified as universal in the file setup.cfg by the universal = 1 key in the [bdist_wheel] section. To publish a universal wheel to pypi:

pip3 install twine
rm -rf dist
python3 setup.py bdist_wheel
twine upload dist/*
git tag v1.0.XXX
git push --tags