The Dept. of Ed. doesn't usually have sysadmins who know how to properly lock down and configure a mail server, in my experience. Neither does Congress, but that's a different story

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Yahoo, Hotmail, AOL, and the rest of the large ISPs are simply trying to enforce the RFCs so that their customers don't get spammed.
Unfortunately, many small business, web hosting, government, and education mail servers aren't maintained by sysadmins who know how to configure such things as antivirus, patch/vulnerability management, anti-spam configurations under Sendmail, Exchange, or your mail server of choice, DNS management, and firewalls.
The larger ISPs and shops that have somewhat of a clue don't have a problem here. The problem is that setting up a mail server safely these days requires a lot of work and knowledge that most sysadmins do not have. The issue is that you have to know areas which were once at least three different domains to at least get AOL to see your mail. And no, I am not kidding about the last part.