That error message appears when Thunderbird is trying to send using the wrong mail server for the account.

Right click the account in the folder pane, select settings

At the very bottom of the list their will be Outgoing server (SMTP) That entry should contain one outgoing server for each mail account.

Note the correct one for the accounts you are using. Now click on the account names in the list and check that the correct outgoing server is set for that account.

What I think is happening is your error are when you get mail, perhaps redirected that you reply to.

So reply to mail sent to [email protected] works, but replies to mail sent to ME@Mydomain com do not as the outgoing server used for both incoming mail addresses is the same.

Thanks Matt for this answer. The settings for outgoing server are in accordance to what the ISP requests (checked with them). I can send email upon starting Thunderbird. Then it opens the connection with the ISP, as normally is for appr. 20min every time you receive messages. After this time, I receive the error when sending. If I set Thunderbird to get messages within 20min, it will leave the gate open. My setting: every 2 min. However, it seems Thunderbird only gets the gate open at the first time connection is made. Question is why this is and how to solve. Other mail programms seem not to have this issue, but I like Thunderbird.

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