Get a mail provider that is capable of delivering a mail service. Yahoo apparently are not and simply ignore all attempts to penetrate to corporate wall. Verizon bought them, merged that mess with AOL and not we have that I am please to say is not my circus.

I would ask Verizon why their mail service is simply broken.

You choices are; 1. Do not leave mail on the server using POP 2. Use IMAP instead of POP 3. Vote with your feet and get a mail provider like GMX that appears to be capable of delivering a simple mail system that works.

I have been encountering this same problem on two different computers running T-bird. I think I've found the solution since its worked on both machines several different times. The simple solution is to delete a bunch of e-mails and restart T-bird, but the underlying problem appears to be associated with the cache usage. Go into Tools/Options, and then select the Advanced tab. Look at the cache usage info, it will probably show that your cache usage is at the limit. Either clear the cache or increase the cache allocation, then restart T-bird.