I am attempting to send a message today to about 1600 recipients. I got about 400 sent, and then suddenly I started getting an error that there was a problem sending and to check my settings. My smtp settings are correct, and a test of the settings (3 times now) always goes through properly.
Any ideas on how to troubleshoot this? I do not see any errors in my logs. It doesn't seem likely (although possible I guess) that any of the ISPs involved in this transaction are blocking traffic, because the tests work.