When logging into webmail (something I hadn’t done for a very long time and I don’t think many others have used it either), I wasn’t able to see mail. Instead I got errors about the redirect.php page. On some browsers, it even tried to download the php file instead of viewing it. Turns out in /etc/squirrelmail/config.php, I hadn’t changed the domain to the new name of the computer. Once I fixed that and restarted http, everything worked fine.