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 "email@example.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.