I had a similar problem with Mail, but it was not my ISP (Spectrum): I could receive emails by using Outlook instead of Mail (Outlook is not my first choice). In addition, I could log into Spectrum's webmail with the same password. The solution was to create a new account in Mail. This was Apple's idea and it has worked.
Several years ago, when I was using MS Entourage, my account became corrupted and I had to do the same thing.
By "create a new account in Mail" do you mean an account with a new e-mail address and password, or just delete the old IMAP account and recreate it as new? The latter is safe to do, the former requiring some (more complex?) transitioning.
