We did a migration of around ~4000 accounts. They sent us a CSV of accounts / passwords and we wrote a C# app that used a tool called imapsync to migrate email (I dont trust the SM migration stuff, had too many issues with it stalling, not working correctly, crashing SM etc. over the years). It worked beautifully. The 3rd party mail server was quite slow, so it took a few days, but it worked without a hitch.