Hello.

Since April 24, GMail has a new policy where they refuse emails containing duplicate headers: To, Cc, Subject, Date, From, Sender, Reply-To, Bcc, Message-ID, In-Reply-To, References. They use rfc5322 as a justification, where multiple occurrences of those fields are considered obsolete syntax.

I'm not sure it's CGP's job to enforce this on outgoing emails, but it sure would be great.

However, the LIST module does generate duplicate headers when the list's  "RFC822 Fields to Keep" setting for To/CC is set to "copy as CC" and a message is received that includes both a To and a Cc address that is not the list.

This:
To: "Someone" <someone@example.com>
CC: "mylist@mydomain.com" <mylist@mydomain.com>,
       Someone Else <someoneelse@example.com>

gets rewritten as this:

To: "mylist@mydomain.com" <mylist@mydomain.com>
Cc: "Someone" <someone@example.com>
Cc: "Someone Else <someoneelse@example.com>

And the message is refused by GMail because of the duplicate CC header.

For now we disabled the Copy as CC setting, but I think CGP should try not to generate a noncompliant message.