Evaluate XCLIENT and CLIENTID extension support
Andrew C Aitchison wrote, on the fetchmail mailing list:
Date: Thu, 23 Mar 2023 10:55:53 +0000 (GMT) Message-ID: 9be0d886-4038-ceca-ffc5-3c277b82f796@aitchison.me.uk
I have become aware of two extensions to SMTP (and one to IMAP too) that may be of interest to fetchmail developers and users.
The first is https://www.postfix.org/XCLIENT_README.html which says that The XCLIENT command targets the following problems: ... ... ... 2. Client software that downloads mail from an up-stream mail server and injects it into a local MTA via SMTP. In order to take advantage of the local MTA's SMTP server access rules, the client software needs the ability to override the SMTP server's idea of the remote client name, client address and other information. Such information can typically be extracted from the up-stream mail server's Received: message header.
XCLIENT is implemented in postfix and is now a wishlist item for Exim: https://bugs.exim.org/show_bug.cgi?id=2702 as well as being available in a fork of Exim ( https://github.com/SpamExperts/exim/commit/3798d48d73c89f7835726d31f096851f7f7fca2a ).
The second is CLIENTID, a pair of draft RFCs https://datatracker.ietf.org/doc/draft-storey-smtp-client-id/ https://datatracker.ietf.org/doc/draft-yu-imap-client-id/ which allows the mail client to tell the server more about itself and the user, allowing a server to restrict access on a device+user basis; eg to block a spammer who has got hold of your login details, whilst continuing to allowing you to send and receive emails from your phone.
CLIENTID is supported by Thunderbird, emClient, and BlueMail clients and at least MagicMail "the Top Selling Carrier Grade Email platform for the ISP, Telco, and Cable industry in North America". I am currently attempting to write CLIENTID support for Exim.