Declare Cypht as the mailer through the User-Agent and X-Mailer email headers

[nekohayo]

🗣 Suggestion

From emails I have received from @marclaporte, I noticed that there seems to be no mail client identification string in the message headers when sent by Cypht.

If that is indeed the case, I think it would be good for Cypht to advertise itself in the metadata of outgoing emails. I suspect some email servers / filters might frown upon the lack of identification otherwise, but also, it makes troubleshooting more difficult.

Examples of such mail headers, that can be seen with Evolution's "View > All message headers" menu action:

• User-Agent: Mozilla Thunderbird
• X-Mailer: Usermin 2.102
• X-Mailer: Microsoft Outlook 16.0
• User-Agent: Evolution 3.51.1
• User-Agent: Evolution 3.54.1 (3.54.1-1.fc41)
• User-Agent: K-9 Mail for Android

As you can see, the industry is a bit of a mix from both (X-Mailer apparently is the old informal standard, and User-Agent seems to be the newer one, as evidenced by its popularity among FLOSS email clients), also observed in this old Thunderbird ticket. I suspect that you'll want to use at least User-Agent, though presumably using both at the same time wouldn't be a bad thing?