welcome here
when you send a message from an external server, you're not using SME and you've got no control over, for example, outgoing mail queue
BTW, open a shell on your server and give
tailf /var/log/qpsmtpd/current
and send a message to your account from an external provider (Gmail, hotmail, $whatever).
take a look at the log scrolling: if you see your message arriving, everything is working.. if it takes some time to see your message, the issue is outside your control