The server returned the error: Mailbox doesn't exist:

I use Mac Mail to manage my domain's email accounts hosted at Network Solutions (IMAP for Incoming Mail Server and SMTP for Outgoing Mail Server). It's been about six weeks since I started consistently getting a pop-up error message from Mail stating: "The server returned the error: Mailbox doesn't exist:". This message pops up about every 30 seconds while I'm trying to compose or send emails in Mail:

This error message began happening at about the same time Network Solutions performed an email server upgrade/migration. I've spent numerous hours with Network Solution's on-line chat support changing various server settings in Mail with no resolution to the issue. They are not taking ownership of the problem and claim that it is "a client-side issue" in spite of the fact that the error message clearly references the "server". I've used the same server settings on my Mac Mail for years and have never had an issue. I've made no changes to my Mail accounts, the only thing that changed was the fact that Network Solutions upgraded/migrated our e-mail server. This was happening on OS Monterey and continues to happen even after upgrading to OS Ventura.

Can anyone please help me resolve this issue? It's really had an impact on my productivity as I need to click the error message pop-up every 30 seconds in order to close it so that I can continue composing an e-mail. Thanks in advance for any help you can offer!