This problem is widespread among Eudora users. My problem also began on early Thursday morning, Matt. The SSL certificate being rejected is owned by Verizon, according to Wikipedia.
I spent at least an hour yesterday afternoon with Daniel, a Verizon tech support guy out of CA. He remotely accessed my system to get familiar with Eudora 7.1. Although Verizon doesn't support Eudora, he is going to download the client, work with it, and contact me at 11:30 AM EST on Monday. I hope he has good news then.
I reported this problem in another thread on Thursday morning, the 27th. Search my userID, Joe63, for details.
Avast users report that its certificate manager has blocked the updated Verizon Certificate. Apparently you have to manually accept it. Hope it works for you.
Something similar may be true for other Network security suites
Problem sending also occurred in 2013. Went back to find it under "Problems sending email":
I've just tried that fix again and it's working.
Find your Eudora.ini file and under "Setting" add:
I had the same issue last week.
SSL Negotiation Failed: Unknown Error
Certificate bad: Destination Host name does not match host name in certificate
But ignoring this error because Certificate is trusted
The connection with the server has been lost.
1. You need add the new SSL certificate to the trusted list. In Eudora, Click Tools > Personalities on the menu.
2. In the personalities window, right-click on the personality and select Properties.
3. Click on the Incoming Mail tab
4. Click on the Last SSL info button.
5. Click on the Certificate Information Manager button.
6. In the Server Certificates area click on the + until you expand/get to the bottom.
The certificate in question may have a skull and cross bones icon. Click that certificate to highlight.
7. Click the Add to Trusted button and OK to close out the screen.
8. I had to do this two times to accept two certificate updates.