Interesting problem today. Bunch of users couldn't send mail to what seemed to be random domains. A little basic investigating showed that they were all hosted on a particular Network Solutions mail server. Seems as though NetSol had a series DoS attacks against them back in August and had blocked entire IP ranges. I could get to the NetSol server from some locations, but not others.
My first call to Network Solutions was fruitless. I spent a great deal of time explaining to the poor tech on the other end that I was not a NetSol customer but someone trying to email a NetSol customer. After great lengths, I was told to configure my Outlook client to use ports 110 and 2525 for communicating. Umm. No.
The second call was much better. I landed at a tech who understood what it meant when I said I could't telnet to this particular mailserver from my netblock. Ticket opened, but I'm sure it will be promptly dropped in the bit bucket.