I'm a new user of FM, and so far pretty excited

Having used Opera 12.17 as my primary eMail client, there's a whole new world to explore with FM now!
I have one issue though: after migrating all my stuff (mail accounts, adress book, feeds etc.), there are a couple of mails that FM inists(!) to mark as spam.
I can if course 'Not spam' them, but this holds only until the next sync or Get Mail action. Then the spam icon appears again.
I already reset the learning spam filter, no success.
I also checked that there are no spam filters in Yahoo or GMX, no success.
I even marked and unmarked a couple other mails as spam, no success.
So please, could you help me and teach me how to convince FM that these are _not_ spam mails?
Thanks a lot, and thanks for this great product!
Win 7 with FM 25.1.9 (x64)