The current public release of MailMate (r5673) mostly works on the latest macOS release (Big Sur). One of the known issues is that printing doesn’t work at all. Apparently, this is used quite a lot by MailMate users (most often to do “Save as PDF” I assume) since I believe I’ve received more feedback on this than any other MailMate issue – ever.
For now, I recommend that Big Sur users upgrade to the latest test release of MailMate. At the time of writing, the latest test release is revision 5757 and it can be fetched by holding down ⌥ when clicking “Check Now” in the Software Update preferences pane of MailMate. Alternatively, here is a direct download link.
The test release does include other Big Sur related changes, but it also differs from the public release in many other areas. For example, both the message view and the composer window have been almost entirely re-implemented. Note that not all of these changes are 100% complete or even documented and this is why it’s not a public release for all users. Users on earlier macOS releases might want to wait until I’m ready to make it a public release.
Thanks to all users and supporters of MailMate! I’m looking forward to continuing my work on MailMate in 2021.