** Help ** exchange wont accept email after an IP chage

Good old NThell did a network upgrade late last week and in the proccess we
lost our "static" IP and were given a new one, after repointing the MX
records we still are not getting email, tried to telnet in via port 25 (to
sbs - not isp mail server) to check everythings okay, but get nada - server
accepts the connection and then instantly shuts it down, nothing stands out
in the logs. Im kinda stuck - NTHell say its my end though it only started
after the IP change, SBS 2003 on a cable modem no ISA

TIA


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