I'm sorry for the vague title. Not sure how better to express the problem.

I have two email accounts associated to Thunderbird, we can call them FIRST@server.com and SECOND@server.com. They are hosted at the same website. I have registered a number of identities to both; the main identity for each is no longer the login identity that I would use, say, for webmail. This is a recent change, at least for SECOND@server.com, because I was previously using the login identity as my main identity.

By "identity", I mean a new address with an alternate "From" and "Reply-to". In Thunderbird, under Account Settings, there is a button called "Manage Identities".

Now, I get the following error when I try to send any email from any of the identities associated to SECOND@server.com:

An error occurred while sending mail. The mail server responded:
5.7.1 : Sender address rejected: not owned by user FIRST@server.com. Please check the message recipient "recipient@recipient.com" and try again.

I have no issues receiving emails, though, just sending.

I am not sure if this a server-side or client-side problem. I have gone to the user settings at the email server and all of the identities are properly set up there for SECOND@server.com. It appears to me that, at least at the front-end, everything is also set up properly in Thunderbird. I lean towards thinking it is a Thunderbird problem though.

  • Can you clarify what you mean by “identity”?  Are you trying to send mail whose “From” line shows something other than FIRST@server.com or SECOND@server.com?  Please do not respond in comments; edit your question to make it clearer and more complete. – Scott Apr 21 '18 at 18:10

You probably have improperly configured SMTP accounts for these identities. For each identity you may choose which SMTP account should be used for sending emails.

Properly configured mail server should no allow you to send email as SECOND@server.com when you're logged as FIRST@server.com. And your error message suggest that this is the case.

To solve it you need to:

  • Configure two SMTP accounts in Thunderbird - one for FIRST@server.com and second for SECOND@server.com.
  • For each identity select correct SMTP account - email addresses of identities should match with SMTP login.
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