lmtp system I/O error

[Will Prater]

List,

I am getting this error all of the sudden, but it is only happening 
from one account. Please make any replies to datafirm at mac.com as this 
address is the one that will not accept mail.

any ideas will really help thanks.

----
Feb  9 23:11:48 stratus postfix/lmtp[95490]: 51562103: 
to=<will at mercurycloud.net>, relay=public/lmtp[public/lmtp], delay=0, 
status=deferred (host public/lmtp[public/lmtp] said: 451 4.3.0 System 
I/O error (in reply to RCPT TO command))
----

--will

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