I've seen several reports on this list of timeout errors with large emails. I'm getting a timeout -- or at least some kind of 'hang' -- every time I send an email. Client-side is running Debian sid, Firefox 1.5.0.1, although I also got the same result with Opera 8.5. The email actually goes through fine, but the "Loading..." translucent box never disappears, nor does the browser ever reload to the message page after sending.
Actually, now I'm trying it on a Windows XP machine, and I get the same result with both IE and Firefox. So is this a server side issue? I don't see anything in the apache error log, or syslog, that suggests a problem.