Is b@a.com sending through the smtp server that has a@a.com's password and User Name? Open Tools/Account Settings, select the b@a.com account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane. If the selection is Default (a@a.com) instead of an smtp server with b's credentials, that may account for what you see.

Hi, it is the same SMTP but they use different Logins. I think its a behaviour problem from Thunderbird itself. Because if I click reply from the Mail on b@a.com the identity dropdown on the top switches to a@a.com - even if the mail resides in the account/folder of b@a.com.

And even more strange: email from a@a.com in account of b@a.com if i click reply the SENDER switches to a@a.com and the RECEIVER to b@a.com ?!?!

Problem still persists on multiple local accounts. Any idea?

The normal behavior is: if you reply to a message in a folder, the From account is the account in which the folder is located. The actual From address may appear different to the recipient if the sender's smtp replaces the original From address with that related to the smtp server. That is why sending accounts and smtp servers should have the same credentials.

You appear to have that set up, so I can't explain your results, unless you have some kind of identity add-on that is malfunctioning.

Actually you can watch firefox switch the from address as far as I can see. The SMTP servers are per account - for each theyr own correct one... I really don't get it...