I found a Plesk 8.3 server running RHEL 4 last month that was presenting errors when users attempted to send e-mail via SquirrelMail: ERROR: Email delivery error Server replied: 127 Can't execute command '/usr/sbin/sendmail -i -t -fsomeuser@somedomain.com'. The error was appearing because safe_mode was enabled and SquirrelMail was unable to drop e-mails into /usr/sbin/squirrelmail. After disabling safe_mode on the server, the users were able to send e-mails via SquirrelMail.