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 like x@y.com, e.g. what you would want in a login form on a website. There are other forms of email addresses, like you would use when composing a message's To: line e.g. My Name <my@address.com>, that this library does not accept. For that try flanker instead.

This library is new and not well tested (and so perhaps not robust) yet, but the goal is to be modern and complete.

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 = "my+address@mydomain.tld"

try:
    email = validate_email(email)["email"]
    # OK, it's valid. Replace with normalized form.
except EmailNotValidError as e:
    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)

For a quick test of the library, you can also run it from the command line:

python3 email_validator.py example@良好mail.中国

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

• Raise 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.

The validator doesn't permit obsoleted forms of email addresses, although 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. If you need validation against the specs exactly, you might like pyIsEmail.

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: accepted mail may still bounce, and bounced mail may indicate a temporary failure (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.

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.

The first is internationalized domain names (RFC 5891). 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 can and should be replaced with its IDNA ASCII form. 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). This library uses IDNA 2008 using the idna module by Kim Davies.

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.

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. 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 should replace the email address with the 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:

email = validate_email(email, allow_smtputf8=False)['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.)

The use of Unicode in email addreses introduced a normalization problem. Different Unicode strings can look identical and have the same semantic meaning to the user, e.g. by using precomposed versus combining characters, glyphs that look alike, and full-width characters. IDNA 2008 (RFC 5895 section 2) requires the following normalizations:

• The domain part of the address is converted to lowercase. Domain names are case-insensitive by design and are necessarily casefolded before encoding in IDNA.
• Fullwidth and halfwidth characters, mostly used in CJK writing, are replaced with their equivalent standard characters in the domain part of the address, as required by IDNA.
• The Ideographic Full Stop character, an alternative to the period (the Full Stop character), is replaced with the period in the domain part of the address, as required by IDNA.
• Unicode "NFC" normalization is applied to the whole address, as required by IDNA and suggested by RFC 6532 section 3.1. This turns characters plus combining characters into precomposed characters where possible. It may also replace certain Unicode characters (such as angstrom and ohm) with other equivalent code points (a-with-ring and omega, respectively).

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

email = validate_email(email)['email']

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

For the email address test@example.org, the returned dict is:

{
  "email": "test@example.org",
  "email_ascii": "test@example.org",
  "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": "example@xn--mail-p86gl01s.xn--fiqs8s",
  "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).

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.
• 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 is 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).
• mx is a list of (priority, domain) tuples of MX records specified in the DNS for the domain (see RFC 5321 section 5).
• mx-fallback is 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).

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.

A handful of valid email addresses are pasted in test_pass.txt. Run them through the validator (without deliverability checks) like so:

python3 email_validator.py --test-pass < test_pass.txt