Hi John,
If I am understanding correctly, you:
1. Have configure SCOT correctly for your email node.
2. You are able to send emails (through what means?) from the system to internal email addresses.
3. You are NOT able to send emails from the system to external email addresses.
If this is the case, I would suggest by checking the transmission request logs in transaction code SOST. Let us know the exact detail of the error from this log.
Kind Regards,
James