when trying to access my email on thunderbird, I get the message: " Login to server mail.sasktel.net with username powalinsky failed".

I get the message "Login to server mail.sasktel.net with username powalinsky failed" even though none of the sign-in credentials have changed and I can access the same account in a browser and on my iphone. This problem began last Friday. It has occurred occasionally and briefly in the past, seemingly resolving itself. I have tried the troubleshooting suggestions that I found online, but to no avail.