For 2 days I have been unable to use Thunderbird on my Ubuntu 18.04 desktop to login to my remote production pop3 server (Postfix, Ubuntu 16.04). Access is denied with message authentication failure; rhost=185.244.41.45. When I do a whois on that ip I find it's in the Russian federation. When I login to Apache on that server from my desktop (Firefox) access is allowed. When I login to apache from my phone using firefox access is denied.
Access to other sites and email addresses on that server are unaffected from desktop and phone. What is going on and how do I stop it?
Thank you
I resolved the problem. On Android, the browsers were checking the domain registration, which expired 2 days ago. On the desktop, only the certificate was being checked. I presume Thunderbird was also checking the registration.