Once this is done, you would expect it to just work.
Unfortunatly, you may get the following issue.
When you create a new email message in SO01, you enter the recpient and the message text, then click send, and you are prompted with an error:
The error in the log book says:
"You do not have a sender address in the chosen communication method."
This was an odd error, but a quick search on SAP notes revealed note 552616.
The note mentioned either setting my user's external email address in SU3 (or SU01), or alternatively, instead of doing this for all users that wish to send external mail, you can set the "default domain" in SCOT.
Set the default domain to something like "mydomain.com".
This means SAP will create an outbound sender address comprised of the SAP username plus the default domain (firstname.lastname@example.org).
I was then able to send mail externally.