[djtremors]

Digiorb,

The best place to start diagnosing problems with non ispconfig related issues is check your /var/log/ files and in this case it's /var/log/maillog.

Start this command and monitor it's output while sending an email.

tail -f /var/log/maillog

capture all that info and post it here if possible.
Also, another command to work with is :

mailq

this will tell you any mail sitting on the server waiting to deliver or is being delivered still (ie large emails).

this is a starting point for you. Also, let us know which mail server you are using, ie Postfix or Sendmail, etc..

PS: Logs are your friend.