[MlMt] mailmate randomly changes the sender address

[Tom]

Hi, I'm a new user.
So far everything is great except for a no-go problem: Mailmate randomly changes the sender address of my messages.

I've set it up with 4 mail boxes:
- yahoo
- gmail
- personal IMAP account (domain A)
- personal IMAP account (domain B)

Quite often, when I send a message using one specific address in the "FROM" field (and now I triple check every time), Mailmate decides to change it with another one and sends the message with from the wrong mailbox.
I've already verified that this is not a local problem (e.g. mixed labels in the GUI): these "swaps" appear on the remote IMAP directories as well.

Is it some sort of hidden option I haven't noticed? Am I doing something wrong?
Can it be fixed?

Thank you for your help.
All the best,
Tom