Michael Tsai, the developer of EagleFiler and the SpamSieve plug-in for Apple Mail on Mac, has written a blog post warning macOS users about potential data loss in Mail when upgrading to macOS Catalina 10.15.0 (build 19A583).
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.
Despite the latter risk, it's still good practice to make backups, but 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.
Tsai says he's unsure whether these issues are due to Mail bugs or to other factors such as problems on the Mac or with the mail server. Apple released macOS Catalina 10.15.1 beta to developers on Friday, but it's still unclear if this version resolves the Mail app bugs. Regardless, Tsai's advice to users who rely on Apple Mail is to "hold off on updating to Catalina for now."
Affected readers can find the full breakdown of the issues here. Have you had problems with Mail since updating to Catalina? Let us know in the comments below.
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