[iMS] Error Message

[Paul Smith]

Yesterday my customer sent mail to one of his lists (list is on his email 
client) fine.  Today, sending to the same list he received this 
error.  What's going on?

best,  paul

ERROR
A message in your Outbox could not be sent.  The server did not recognize
the recipients.
EXPLANATION
Mail Rejected
error: 5550

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