How do I clear the Mail cache in High Sierra?

I exclusively use Mac Mail with no issues until suddenly yesterday when I sent a client with whom I had not had any email contact with for several months, an invoice which was rejected by their server.

The attachments were not overly large, approx 3M total.

The Delivery Failure notice said "Reason: Remote host said: 554. Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means."

(The recipient, when I called, had no idea why it was happening.)

I asked someone on my end, who said my email might be trying to use our old IP address instead of our new one that is currently in place. I was told to try flushing my cache...

So, how do I do that? None of the posts I see tell how to specifically flush or clear the mail cache in High Sierra. None of the fixes that I see are relevant to the High Sierra setup. I really don't want to clear all my caches across the board.

Any suggestions??