Hmmm. My problem does seem similar if not exactly the same - see description, below.

So is my mail provider rejecting my other return email addresses because it is not the "official" one connected to my account? I don't get it when I am only using the return email address when sending through my provider's system.

Is there any way around this?

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I am having an irritating Thunderbird problem that I can’t figure out how to fix. Any suggestions would be appreciated. In the past I have used different email addresses – for different purposes as my return email address. I would just click on the dropdown and I would then click on the one I wanted to use and then send the email – no problem. Of the three return email addresses only one of them was the same address that appears as my account name address that appears when I open “Account Settings.” In “Account Settings” I use one of my alternative email addresses in both “email address” and “reply-to address.” Recently, when I try to use one of the other two email addresses, and I click “send” I am getting the following message:

An error occurred when sending mail. The mail server responded you are not authorized to send mail as: ashefman@theedgequality.com. Please verify that your email address is correct in your account settings and try again.

When I then switch to using my account settings name address the email is accepted.

So what can I do to allow me to use all three email addresses at any point?

I have 2 accounts set up in Thunderbird. However, irrespective of number of accounts set up, Thunderbird uses a single outgoing server. The server could be the same for any outgoing email from any account. Incoming server needs separate authentication (password) for each email account. I never had this problem with thunderbird before.

The toc.santos account is sent by the smtp toc server.