Since this is the third complaint (but only 2 people) have reported this (all day) I am going to go ahead and make a public post with the assumption that more people are probably affected. The server isn't down. The certificate was auto-renewed last night. The certificate is from Let's Encrypt which changed their intermediate certificate recently so the new cert is signed by a different cert than before.
Of course obsolete systems that can't handle modern security also can't handle the new intermediate certificate. At least not without some help. We can't promise that this will help or tell you which email clients it will help in but the new intermediate certificate is available at
https://letsencrypt.org/certs/lets-encrypt-r3.pem It should be possible to add it to an email client and make things work again.
The only other suggestion is to use our web mail client QuestMail exclusively.
Or of course that it is time to upgrade
BTW, I did respond to your ticket with this information but if you can't read your email you probably didn't know that.
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