EmailMessage.message() should not set a blank To: field when EmailMessage.to is empty

[Jon Dufresne]

EmailMessage.message() always sets a To: header, see:

​https://github.com/django/django/blob/a9e5ac823df8ba8b786b6450c967ca378c008d0e/django/core/mail/message.py#L257

In my project, emails are sent to Cc: or Bcc: recipients without a To: recipient.

Some mail backends are very strict about allowed headers and their values. For example, Amazon Simple Email Service (SES) validates all recipient headers to contain valid email addresses. It fails without delivery if any header contains an invalid address. For some details, see:

​https://boto3.readthedocs.io/en/latest/reference/services/ses.html#SES.Client.send_raw_email

The message must include at least one recipient email address. The recipient address can be a To: address, a CC: address, or a BCC: address. If a recipient email address is invalid (that is, it is not in the format UserName@[SubDomain.]Domain.TopLevelDomain ), the entire message will be rejected, even if the message contains other recipients that are valid.

SES considers blank as invalid. When sending an email serialized by EmailMessage.message() to Cc: or Bcc: only addresses, SES fails with an error. To: avoid the error, Django can avoid setting the To header when EmailMessage.to is empty.

[Jon Dufresne]

​PR

[Claude Paroz]

Makes sense.

[Tim Graham <timograham@…>]

In da82939:

Fixed #28912 -- Made EmailMessage.message() omit an empty To header.

[Sarah Boyce <42296566+sarahboyce@…>]

In b909853:

Fixed #35033, Refs #28912 -- Fixed repeated headers in EmailMessage.

Fixed a regression which would cause multiple To, Cc, and
Reply-To headers in the result of EmailMessage.message() if
values were supplied for both to/cc/reply_to and the
corresponding extra_headers fields.

Updated related tests to check the generated message() has
exactly one of each expected header using get_all().

Regression in b03d5002955256c4b3ed7cfae5150eb79c0eb97e.