There is a “Connection Doctor” when activated from Apples Mail app, from menu window, this will show information on things such as;
When it’s checking for mail
Sending mail
downloading 1 of X emails
it will also report if it’s having connection issues with connecting to your incoming/outgoing email server
If your seeing green leds for your email servers, that’s good
if there are yellow/red time to call your email provider and ask if they are experiencing email server issues. Also check with others if there are having the same issue.
You can have a situation where one might be able to get email, but not send. This is indicated by email setting in Outbox, which means it didn’t send right away and will try again.