The servers in question both run Domino 9.0.1.FP4 and have done so since October/2015.
I don't apply fixpacks unless there is an issue, because if it ain't broke, don't fix it.
I do not have SSL enabled on these two servers.
I cannot duplicate the issue that has been experienced.
It is an intermittent issue, apparently, that can affect senders from whom we have previously received emails.