Normalize some mailbox names like postmaster to lowercase per RFC 2142

CHANGELOG.md

@@ -12,6 +12,7 @@ There are no significant changes to which email addresses are considered valid/i
 * The quoted-string local part syntax (e.g. multiple @-signs, spaces, etc. if surrounded by quotes) and domain-literal addresses (e.g. @[192.XXX...] or @[IPv6:...]) are now parsed but not considered valid by default. Better error messages are now given for these addresses since it can be confusing for a technically valid address to be rejected, and new allow_quoted_local and allow_domain_literal options are added to allow these addresses if you really need them.
 * Some other error messages have changed to not repeat the email address in the error message.
 * The `email` field on the returned `ValidatedEmail` object has been renamed to `normalized` to be clearer about its importance, but access via `.email` is also still supported.
+* Some mailbox names like `postmaster` are now normalized to lowercase per RFC 2142.
 * The library has been reorganized internally into smaller modules.
 * The tests have been reorganized and expanded. Deliverability tests now mostly use captured DNS responses so they can be run off-line.
 * The __main__ tool now reads options to validate_email from environment variables.

README.md

@@ -311,7 +311,8 @@ 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.
+normalized to condensed form. [RFC 2142](https://datatracker.ietf.org/doc/html/rfc2142)
+also requires lowercase normalization for some specific mailbox names like `postmaster@`.
 
 Examples
 --------

email_validator/rfc_constants.py

@@ -43,3 +43,10 @@
 LOCAL_PART_MAX_LENGTH = 64
 DNS_LABEL_LENGTH_LIMIT = 63  # in "octets", RFC 1035 2.3.1
 DOMAIN_MAX_LENGTH = 255  # in "octets", RFC 1035 2.3.4 and RFC 5321 4.5.3.1.2
+
+# RFC 2142
+CASE_INSENSITIVE_MAILBOX_NAMES = [
+    'info', 'marking', 'sales', 'support',  # section 3
+    'abuse', 'noc', 'security',  # section 4
+    'postmaster', 'hostmaster', 'usenet', 'news', 'webmaster', 'www', 'uucp', 'ftp',  # section 5
+]

email_validator/validate_email.py

@@ -2,7 +2,7 @@
 
 from .exceptions_types import EmailSyntaxError, ValidatedEmail
 from .syntax import validate_email_local_part, validate_email_domain_name, validate_email_domain_literal, get_length_reason
-from .rfc_constants import EMAIL_MAX_LENGTH, QUOTED_LOCAL_PART_ADDR
+from .rfc_constants import EMAIL_MAX_LENGTH, QUOTED_LOCAL_PART_ADDR, CASE_INSENSITIVE_MAILBOX_NAMES
 
 
 def validate_email(
@@ -92,6 +92,15 @@ def validate_email(
     ret.ascii_local_part = local_part_info["ascii_local_part"]
     ret.smtputf8 = local_part_info["smtputf8"]
 
+    # Some local parts are required to be case-insensitive, so we should normalize
+    # to lowercase.
+    # RFC 2142
+    if ret.ascii_local_part is not None \
+       and ret.ascii_local_part.lower() in CASE_INSENSITIVE_MAILBOX_NAMES \
+       and ret.local_part is not None:
+        ret.ascii_local_part = ret.ascii_local_part.lower()
+        ret.local_part = ret.local_part.lower()
+
     # Validate the email address's domain part syntax and get a normalized form.
     is_domain_literal = False
     if len(domain_part) == 0:

tests/test_syntax.py

@@ -413,6 +413,11 @@ def test_email_test_domain_name_in_test_environment():
     validate_email("[email protected]", test_environment=True)
 
 
+def test_case_insensitive_mailbox_name():
+    validate_email("POSTMASTER@test", test_environment=True).normalized = "postmaster@test"
+    validate_email("NOT-POSTMASTER@test", test_environment=True).normalized = "NOT-POSTMASTER@test"
+
+
 # This is the pyIsEmail (https://github.com/michaelherold/pyIsEmail) test suite.
 #
 # The test data was extracted by: