While I never received the particular error message you were getting, I did
have considerable trouble getting Sympatico to play nice with Thunderbird.
For anyone else who might come looking for help with general Sympatico settings here, these are the settings that ultimately worked for me:
Tools > Account Settings > Server Settings:
Server name: pophm.sympatico.ca Port: 995
User Name: firstname.lastname@example.org (whatever your address is)
Security Settings - Use secure connection: SSL
(no check mark in Use secure authentication)
Tools > Account Settings > Outgoing Server:
Description: <not specified>
Server Name: smtp1.sympatico.ca
User Name: b1xxxxxx (whatever your b1 number is)
Secure Connection: TLS (if available)
At various times, I've had to input both my Sympatico Access Password and my Sympatico email password - when asked for a password, I usually try one, and if that doesn't work, then I try the other. If neither works, I reboot the computer - after that, I've always been successful.