re :I added two mail accounts A and B to Thunderbird and deleted the outgoing smtp server settings of B. A's one is displayed as the default outgoing server in the settings.

Do I understand this correctly: you have selected to use the SMTP settings of account 'A' in the Outgoing SMTP setting of 'B' account.

re: Yet, I'm still able to select B as an outgoing server from the dropdown when composing a new message. This sounds like you are selecting an email address, not an smtp server.

re: I wouldn't mind, but the the smtp server of B is selected by default when I have a folder of the B account selected and start writing a new e-mail. This is selecting the email address of the account, not the outgoing server.

If the Outgoing Server SMTP settings for account B are set up to use Ougoing Server SMTP settings of account A then providing the server accepts sending of email address B via A then this is what is occuring.

I have an email address called 'X' but in order to send I have to use my ISP default outgoing server SMTP which is a differnt email address called 'Z'. In the Account Settings for mail account/email address 'X', I have selected the Outgoing server SMTP of account 'Z'. I set up ISP email address 'Z' to allow sending using a differnt email address called 'X'. When I compose an email, it will use the accounts email address 'X', but when sent it actually goes via 'Z' account. So if I send a test email using 'X' email address to the 'Z' email address, I can check the headers of the incoming email in 'Z' Inbox and it will show the 'X' email address going via 'Z'.

Ah okay, thanks for the explanation. Can I set a default reply address for all of my accounts then? So that account A is used by default for every new message I compose even if account B is selected in the "inbox panel" where all the folders are listed. Cheers, Stefan

Set the reply-to address: in account settings to whatever you want.