Is [email protected] sending through the smtp server that has [email protected]'s password and User Name? Open Tools/Account Settings, select the [email protected] account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane. If the selection is Default ([email protected]) 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 [email protected] the identity dropdown on the top switches to [email protected] - even if the mail resides in the account/folder of [email protected].

And even more strange: email from [email protected] in account of [email protected] if i click reply the SENDER switches to [email protected] and the RECEIVER to [email protected] ?!?!

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...