I honestly don’t know how long it’s out for. When it happens, Apple Mail offers to use a different SMTP server, and I choose to switch to Comcast’s. This choice sticks until I restart Mail, so the failure could be a single rejection and I wouldn’t know.
This problem also happens frequently to people at my church, whom I moved to DH for web and mail. They’ve complaining to me about their frustration with this about once a week for the last couple of months. They’re on the same SMTP server.
Contrast this to Pair, where we were before. SMTP failures never once happened in the four years we used them.
I could put a logger on my mail client to see exactly what happens when I get a SMTP failure. Would that be useful, or do you already log SMTP failures?