I have my own mailserver, it used to be hosted by Hostgator but now I have a VPS.
From my home location I can send and receive mails as much as I want. It even works on my phone.
My other users also are able to login to their accounts when they're at my house, on my connection.
However: when they go home they can't. And I have no idea why.
So, TL;DR: When my friend sends a mail from his home thunderbird throws an error, when he does it from here (on his laptop) it works fine. The server is in England.
I know this isn't much to go on, but I have no clue where to start on this. I haven't set any host specific filters.