Michael Tsai, the developer of the long-standing SpamSieve plug-in for Mail on the Mac, has written a blog post warning about potential Mail-related data loss when upgrading to macOS 10.15 Catalina. In particular, he’s hearing from numerous customers that updating Mail’s data store from Mojave to Catalina sometimes says that it succeeded, when in fact, large numbers of messages turn out to be incomplete or missing entirely. Plus, moving messages between mailboxes on the Mac can delete message bodies, leaving only headers. If you moved the message to a mailbox on the IMAP server, other devices see it as deleted, and that deletion status eventually syncs back to the original Mac, where the message disappears as well.
As Michael Tsai points out, these problems can be difficult to notice (if you’ve already updated to Catalina, run through your mail to see if it has been affected), the problems can propagate to other devices, it’s difficult to make a complete backup, and even if you have a backup, it’s hard to restore.
To reiterate our advice in “macOS 10.15 Catalina Ships, Upgrade with Caution” (7 October 2019), hold off on upgrading to Catalina for a while, particularly if you use Mail.