I believe I have solved the problem
It appears that the issue was with Outlook and authentication.
After clearing login information, closing outlook, logging off, logging back in and reconfiguring account login information in outlook.
The problem now appears to be resolved and emails are being delivered to addresses that previously were getting relayed denied.
The odd thing was that emails sent to users setup locally on the SME server domain (user@3xgmm.com) were being delivered, however external domains were randomly being denied.
If the outlook smtp login details were wrong then no emails should have been delivered at all, because smtp sme server should have rejected users connection ?
Users computer and SME server are NOT local or connected through local interface.
I will report back if the problem occurs again.