Fixed #25986 -- Fixed crash sending email with non-ASCII in local part of the address.

On Python 3, sending emails failed for addresses containing non-ASCII
characters due to the usage of the legacy Python email.utils.formataddr()
function. This is fixed by using the proper Address object on Python 3.

django/core/mail/backends/smtp.py

@@ -115,9 +115,9 @@ class EmailBackend(BaseEmailBackend):
         """A helper method that does the actual sending."""
         if not email_message.recipients():
             return False
-        from_email = sanitize_address(email_message.from_email, email_message.encoding)
-        recipients = [sanitize_address(addr, email_message.encoding)
-                      for addr in email_message.recipients()]
+        encoding = email_message.encoding or settings.DEFAULT_CHARSET
+        from_email = sanitize_address(email_message.from_email, encoding)
+        recipients = [sanitize_address(addr, encoding) for addr in email_message.recipients()]
         message = email_message.message()
         try:
             self.connection.sendmail(from_email, recipients, message.as_bytes(linesep='\r\n'))

django/core/mail/message.py

@@ -13,7 +13,7 @@ from email.mime.base import MIMEBase
 from email.mime.message import MIMEMessage
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
-from email.utils import formataddr, formatdate, getaddresses, parseaddr
+from email.utils import formatdate, getaddresses, parseaddr
 from io import BytesIO
 
 from django.conf import settings
@@ -103,22 +103,66 @@ def forbid_multi_line_headers(name, val, encoding):
     return str(name), val
 
 
+def split_addr(addr, encoding):
+    """
+    Split the address into local part and domain, properly encoded.
+
+    When non-ascii characters are present in the local part, it must be
+    MIME-word encoded. The domain name must be idna-encoded if it contains
+    non-ascii characters.
+    """
+    if '@' in addr:
+        localpart, domain = addr.split('@', 1)
+        # Try to get the simplest encoding - ascii if possible so that
+        # to@example.com doesn't become =?utf-8?q?to?=@example.com. This
+        # makes unit testing a bit easier and more readable.
+        try:
+            localpart.encode('ascii')
+        except UnicodeEncodeError:
+            localpart = Header(localpart, encoding).encode()
+        domain = domain.encode('idna').decode('ascii')
+    else:
+        localpart = Header(addr, encoding).encode()
+        domain = ''
+    return (localpart, domain)
+
+
 def sanitize_address(addr, encoding):
+    """
+    Format a pair of (name, address) or an email address string.
+    """
     if not isinstance(addr, tuple):
         addr = parseaddr(force_text(addr))
     nm, addr = addr
+    localpart, domain = None, None
     nm = Header(nm, encoding).encode()
     try:
         addr.encode('ascii')
-    except UnicodeEncodeError:  # IDN
-        if '@' in addr:
-            localpart, domain = addr.split('@', 1)
-            localpart = str(Header(localpart, encoding))
-            domain = domain.encode('idna').decode('ascii')
+    except UnicodeEncodeError:  # IDN or non-ascii in the local part
+        localpart, domain = split_addr(addr, encoding)
+
+    if six.PY2:
+        # On Python 2, use the stdlib since `email.headerregistry` doesn't exist.
+        from email.utils import formataddr
+        if localpart and domain:
             addr = '@'.join([localpart, domain])
-        else:
-            addr = Header(addr, encoding).encode()
-    return formataddr((nm, addr))
+        return formataddr((nm, addr))
+
+    # On Python 3, an `email.headerregistry.Address` object is used since
+    # email.utils.formataddr() naively encodes the name as ascii (see #25986).
+    from email.headerregistry import Address
+    from email.errors import InvalidHeaderDefect, NonASCIILocalPartDefect
+
+    if localpart and domain:
+        address = Address(nm, username=localpart, domain=domain)
+        return str(address)
+
+    try:
+        address = Address(nm, addr_spec=addr)
+    except (InvalidHeaderDefect, NonASCIILocalPartDefect):
+        localpart, domain = split_addr(addr, encoding)
+        address = Address(nm, username=localpart, domain=domain)
+    return str(address)
 
 
 class MIMEMixin():