The alert says "An email did not reach every address"

The mail server took a message for some of its addresses and refused the others, usually an old address in a Cc or a Bcc, or one typed wrong. Email is still going out, and the addresses it took have the message, so it is not tried again: that would send them another copy each time. In Email Delivery → Log, open the message marked Partly sent (or Refused, when the refused address was the one on the To line), and read which address the mail server refused. Put the address right where it comes from (WooCommerce's email settings for a shop copy, the order for a customer), then use Send again on the message's page, which starts with only the refused addresses.

This alert is counted apart from the one that says email is not going out, so it never holds that one back. If the mail service stops taking anything an hour later, you are told at once, and the notice turns red.