In late Jan. - early Feb. 2020 we had long discussion of this topic. The final suggestion I received was to perform that operation via a lengthy procedure of converting each and every mail folder ("MAILBOX" in Apple's terminology) to EML format. I wanted to migrate abt 30,000 emails grouped in over 300 "Mailboxes", being an archive of more than 20 years. The job seemed to be monumental and I then put it on the back burner.
Last month I upgraded my 2013 MacBook Pro to a 2020 one, running on OS 11 (Big Sur), which does not support SM. Migrating to Apple MAIL became a must.
I think I should report my experience to our members.
I firstly converted all SM folders to Apple Mail folders ("Mailboxes" as I wrote) on my old computer. That was done painfully slow, folder by folder and took me 35 net working hours. The results were excellent, not a single email of the 28,975 I had was lost. I then migrated the newly-converted Apple Mail MAILBOXES from old computer to new. It took abt 4 hours.
To my surprise (and frustration) when I opened Apple Mail on the new computer, all Mailboxes were found to be empty and none of my emails showed up. The directory, however, had indicated the presence of mail, to the amount of 12.65 GB.
A 90 minute call to Apple Support toll-free number, and a very dedicated member of the support team, solved the problem.
All my mail is now up and going under Apple Mail's banner, which seems strange and very much different to SM. It will take some time to get used to it. Unfortunately that was my only way to stay in the eMail business, especially when nobody could predict if and when SM will become compatible with Big Sur.
This note was NOT written for the purpose of encouraging its adoption by any of our members. The procedure consumed many hours of hard work, sometimes quite frustrating.
