I don't know what .msf stands for, but those are header files. They are rebuilt, if deleted, basing in the equivalent file with no extention. The .msf files cannot be used, unless combined to the files with no extention. None of the 65 .msf files are useful by themselves. You can, safely, delete them from Windows Explorer, with your program closed. They will be rebuilt.
I have no idea what those 65 files are associated to. I thought that I had a lot of accounts (12), and I don't have that many files. And, you should not count Trash.msf, Junk.msf. You should, specifically look at the Inbox.msf, or maybe Sent.msf, which more likely to become corrupted.
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