What you need to know
- Report suggests Catalina may be experincing data loss issue within Mail App
- Moving emails between inboxes can result in blank messages which are eventually deleted.
- Data store failing to pull large numbers of messages from Mojave to Catalina.
According to the report, users on macOS Catalina 10.15.0 (build 19A583) are reporting problems when updating Mail's data store from Mojave to Catalina, and when moving messages between inbox.
According to Tsai, he's heard from several users that updating Mail's data store from Mojave to Catalina sometimes says that it has succeeded, when in fact on closer inspection it turns out that large numbers of messages are incomplete or missing entirely.
In addition, users have reported the loss of message content when moving emails between mailboxes. From Tsai's post: Moving messages between mailboxes, both via drag-and-drop and AppleScript, can result in a blank message (only headers) on the Mac. If the message was moved to a server mailbox, other devices see the message as deleted. And eventually this syncs back to the first Mac, where the message disappears as well. Tsai warns that these issues are particularly pernicious because users may not realize anything's wrong unless they look at affected messages or mailboxes. Since the data is synced to the server, these problems can also propagate to other computers and devices, and relying on backups is difficult because Mail data is continually changing and there's no easy way to merge restored data with messages received since the last backup.
So not only is data store failing to recover large numbers of messages, moving messages between inboxes appears to be turn some emails into blank messages. When devices on your iCloud see these blank messages they delete them, which in turn results in the message being deleted on your Mac as well.
The report notes that the best defence against this currently (apart from not updating to Catalina) is to ensure you have a Time Machine backup of all your Mail Data.
Tsai notes that Apple Support appears to be erroneously advising users that lost Mail data in Catalina can't be recovered from a Time Machine backup made using macOS Mojave. According to Tsai, this is not the case: Apple Mail's File -> Import Mailboxes... menu bar option can be used to selectively import them into Mail in Catalina as new local mailboxes.
It is unclear at this stage whether Catalina 10.15.1, released in beta form on Friday to developers, has resolved this issue. In the meantime, anyone who relies on Apple's Mail App is strongly advised not to upgrade to macOS Catalina.