JMAP ( https://jmap.io/ ) is a (newish) modern open standard for E-Mail that can replace IMAP and SMTP (in communication with Mail-Clients) as well as support for calendaring, contacts data and sieve filters etc. in a similar manner as the proprietary MS Exchange protocol.

It is used and developed in large by fastmail but there are a bunch of Projects aiming to implement it or already have - in particular the Team from MZLA (Thunderbird) announced that their (completely new) iOS Version of Thunderbird will start with JMAP Support (and will add IMAP only later!) as well as their Thundermail-Service. ( https://blog.thunderbird.net/2025/05/thunderbird-for-mobile-april-2025-progress-report/ )

As we all know you are currently rewriting most of SOGos code base so now would be a good time to think about also supporting this new and modern standard - also are already existing libraries you can base this support on.