Hello @Jim M
I am currently running version 4.7.18 of the software.
I have applied the additional patches available for this version, but I suspect they may not address this specific issue, as it seems unrelated.
I can not repro this issue myself, I will wait 72 hours to see if issue continue, since it's a prod server, we will know.
We dont use any external email service, we have our own email server.
Out of curiosity, what does this error correspond to?