OT - Help with resulting error message

[Steve Campbell]

One of our users is trying to email users at a particular domain and is 
receiving the following error message back from them:

(reason: 550 5.7.1 rejected content, black listed twitter.com by
multi.surbl.org #762 #895 (l51Ehj043192426600))

I asked if he was posting through twitter, and he said he was not. I checked SURBL and our IPs are not listed. So I'm confused(as always) by what this is telling me. He does have a reference to his twitter stuff in his signature. Do any of the SURBL lists really block content?

A good explanation would be helpful. Thanks much

Steve Campbell