Thanks, Jonathan. No, I have the accounts set as POP accounts. This error is intermittent, so I'm guessing that my mail server sometimes times out and sometimes doesn't. Why my Outlook interprets that as an IMAP issue, I don't know.
Would you recommend that I contact my email account administrator in order to get their assistance? (They're really a domain administrator, but my domain is parked and I just use it to host email addresses.)