We're having some problems with 44net email to and from hotmail.com. Mail from that domain is getting delayed, sometimes for hours, and mail going to addresses in that domain is received by the server but not delivered to the users mailbox.
I sometimes have similar problems with gmail.com. I send replies to allocation requests to the user's gmail address and they never receive it, and become impatient and repeat the request. Sometimes I can fix it by using a different sender mail address.
It looks like those big guys (who of course get billions of spam messages a day) use a lot of sender profiling and low-volume senders like @amsat.org and @*.ampr.org get low in their reputation score. So with a *.ucsd.edu sender address you had a quite higher reputation score. Mentioning IP addresses in the mail body does the rest. (often considered an indication of spam)
Rob