I tried changing my Admin's email address, and I got the following error:
Your request was not sent due to the following error: The server rejected one or more recipient addresses. The server response was: 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
Is this an error in the forum code? Does it have something to do with the fact that I'm trying to change the Admin email address?
It most likely means your host does not allow relaying. Usually the domain name portion of the from email address must be a valid domain name thats hosted on your server with that host.
Some hosts allow you to use SMTP Authorisation to allow relaying, but basically you'll need to speak with your host about it and find out just what they allow/disallow you to do with respect to mail relaying through their servers.