Like I said, I am still in the woods. My testing was the direct result of an MS response to a Windows Live Mail user who was having problems with WLM disconnecting. Its advice was to ping at various packet sizes, find one that worked, then set his local MTU to that size -- not that size + 28. Author was Gerald_G, a Microsoft Forum moderator.
The part of the problem I am having that baffles me is that the failure to get notices happened simultaneously on two e-mail accounts running different email software with different OS's (WLM with Win7 and Outlook Express with XP Pro) on two different machines. Each machine was connected to a different ISP (Armstrong, Cox), and the locations are 42 miles apart. It not only affected the e-mails forwarded to the local machines, there is no trace of anything on the corresponding Webmail accounts.
I will call at least one of the providers today and see if it will look in the logs.
John