Maybe if you post your server settings, someone here may have suggestions,

In Server Settings>>Security Settings I changed Security settings from StartTLS in dropbox to None.Receiving emails now.Was sending emails ok all the time. Worked ok for 12 years .Don't know what was going on here. Muddled thru another crisis hopefully. Tnks for your interest David.

Ports may be part of it. If this is an IMAP account, try SSL/TLS and 993 or STARTTLS and port 143 . What you have works, as you say, but without security.

I had exactly the same problem, could send emails but not receive any. I just kept getting that same message as in the opening post. My security setting was SSL/TLS so I changed it to None but it made no difference so I went to Start TLS with still no change so I went back to the original setting, SSL/TLS and guess what, I GOT MY EMAIL BACK. I have no idea why or how but there must have been some glitch somewhere that is now gone. Thankyou to all who have posted because I would have had no idea on where to even start tying to fix this problem and obviously still don't has any idea on how it was fixed but FIXED IT IS.