Hi
Welcome to the forums.
Well, it's not a surprise that you got that one caught as spam. It being sent from a residential dynamic IP & you also appear to be checking at sorbs. It's possible (but you'd need to check this) that your IP block is marked as a spam sender. Have you tried hitting the 'not junk' button in zimbra to train this message as normal? I also don't use blacklists as they tend to give too many false positives such as this. You could try removing those checks for dynamic IP & SORBS to confirm that's the only problem.