SMTP Connections Rejected

We have a dedicated server running WHM/cPanel. Last week this server (which is in a data center, outside our office) started rejecting SMTP connections from computers outside our office, but allowing connections from our office. We do not have a firewall in place. The message returned from Thunderbird when attempting to connect from outside our office via SMTP was a generic "10060" error.

I did notice that if you go into the config on Thunderbird on a computer outside our office and set it to connect using SSL, it then works just fine. I'm finding this baffling and the logs that I see aren't showing any odd activity or providing a reason for the refusal of the connection.

I also don't understand why it accepts connections from our office, as I would have never changed anything to make it accept only from our office, as we connect from home and other locations as well.