Error Number: 451

Hi guys, My client recently upgraded the kernel to 2.4.7 , before that qmail+vpopmail+squirrel mail were working fine.now after upgrading we get this error message ERROR Error Number: 451 Reason: Requested action aborted: error in processing Server Response: 451 See http

RE: Error Number: 451

My client recently upgraded the kernel to 2.4.7 , before that qmail+vpopmail+squirrel mail were working fine.now after upgrading we get this error message ERROR Error Number: 451 Reason: Requested action aborted: error in processing Server Response: 451 See http://pobox.com

RE: Error Number: 451

this error message ERROR Error Number: 451 Reason: Requested action aborted: error in processing Server Response: 451 See http://pobox.com/~djb/docs/smtplf.html. MAIL NOT SENT ... What r the things i need to check to solve this problem ?? can anyone give pointer's,suggestions