SquirrelMail 1.4.5 – Error when sending msg

I'm running the latest version of Mac OS X server 10.4.9 When me or one of my users try to send an email, we get this message : ERROR: ERROR: Bad or malformed request. Server responded: Missing required argument to Append command The email is sent successfully but this error message is kind of scary for everyone. Is this error message ever happen to someone here? Thanks, Rémi