Question : SMTP Protocol Returned a Permanent Error 550 5.71 Space

I have a client that is receiving intermittent mail problems sending to my location they receive a "SMTP Protocol Returned a Permanent Error 550 5.71 Space"  in their Delivery Failure Report.  They are sending from a Lotus Notes/Domino setup.  I've never seen an error with just the word "Space" at the end of it.  Any idea what could be causing this? All I was given was the "SMTP Protocol Returned a Permanent Error 550 5.71 Space" in the Delivery Failure Report.

Answer : SMTP Protocol Returned a Permanent Error 550 5.71 Space

it seems to be somehow related to email relaying. 550 mean -

 If all lookups complete successfully and no matching username is found, the SMTP server returns a 550 permanent failure response indicating that the user is unknown.

for more info look at here http://www.scribd.com/doc/2199950/Domino page 241-2 . it seems, as they sending that email, to non-existent account in your directory.