That is really weird.
We have a "Test Connection" feature in the realm of OCI ASUP settings, under "Setup". You could try to:
Temporarily switch OCI ASUP to "email"
Click "Test Connection"
If it succeeds, OCI is talking to a mail server that accepts its messages.
If it fails, reading the bottom of
May be interesting.
I will file a bug on this (I think this ought not to be a RFE, because I think we used to have a test button)