Thanks Matt for more leads and information so I can better understand what is happening.
This is how it went on my end today. - I started up my computer (only one on here) and got 2 emails from 2 yahoo groups. I wrote an email to my wife ( 1 sentence & 1 link) and it wouldn't send. I am using outlook (up to date). My wife uses outlook express (also up to date). We are both on different accounts under my reseller plan. When one of our emails is down, both of our email accounts don't work. Bottom line is that I got my email once today, and never did get to send even my first.
I rebooted my satellite modem and luckily got a different IP so now both our emails are working fine. I am trying to go through your list of reasons. Do you know if Outlook & outlook express send a quit? Also I'm not sure about reason #1. I'll have to look up those acronyms.