Mail functionality in WikiSuite is handled as follows:
Please see general component criteria and the other Software Components for WikiSuite.
Why Cyrus IMAP and Cypht instead of the many options in this space? Very good question. One important reason: JMAP!
SOGo is a very interesting solution, and was a strong candidate. However, no JMAP support, and it would have not been as tightly integrated as the chosen solution. SOGo is written in Objective-C (vs Cypht in PHP)
Horde is a fully Open Source solution that has been around for a long time. And goes beyond just email with many other apps, including a wiki. And it works with Cyrus IMAP. This is a good solution. However, in the context of WikiSuite, we'll end up with a more integrated solution on our chosen path.
Thunderbird is a desktop application. It can be used with WikiSuite by connecting via IMAP. However, we need a web-based solution as the main solution.
(to do: add all the other options)