Try changing from the Sky servers to the Yahoo servers:

https://support.mozilla.org/en-US/questions/1278588

sfhowes said

Try changing from the Sky servers to the Yahoo servers: https://support.mozilla.org/en-US/questions/1278588

Thanks this worked for me. The only trouble was I had to delete the account before I could add it with the new details and I lost all my mail filters. As the folders were still intact I think I have almost reinstated them from the details in the folders and can fill in any missing bits later.

Thanks again

Filters are stored in the msgFilterRules.dat file in each account subfolder of the Mail (POP accounts) or ImapMail (IMAP accounts) in the profile folder. The dat file can be copied from one account to another, and the deleted account subfolder is probably still there in the profile folder. Help/Troubleshooting, click Open Directory to open the profile in your file manager; close TB before making any changes.