Major software updates are often accompanied by one or two bugs, which seems to be the case with macOS Catalina. Some users report that e-mail messages disappear in Apple Mail. As AppleInsider reports, the problem lies in moving messages and synchronizing them with other devices. In some cases, emails may seemingly go completely.
Other problems can occur when saving emails. Mail does not always download all the necessary message information. Difficulties can also arise when restoring data from an email archive.
Michael Tsai, who developed the EagleFiler and SpamSieve plugins for Apple Mail under macOS, described some of the issues that arose in a blog post. The bugs have been fixed. What is not clear is how many people are affected by the bugs described by Tsai: according to the publications of the forum, it seems that the number of users who have lost emails is relatively low.
The beta versions of the software have been available for four months. It should also be noted that Tsai indicates that the bugs could be related to servers operated by Apple, or more generally to the Mac, rather than to Apple Mail.