Outside of squirrelmail can you send email using just sendmail (such as with the mail command, or something like kmail, sylpheed, mutt, evolution, or any other mail client)? Are you having any other network problems? Can you post up your sendmail.cf (I believe that's what it is, I use postfix and love it
) so we can see if it's a configuration problem? Are you using an IMAP server? Which one? What distro? How did you install these things? Are you running your own DNS server?