ms outlook error message after sending, but everything still goes thru fine

[Susan]

this is the message after clicking send/receive:
could not send 'cause recepient was rejected by server
server response 501 syntax error in parameter or arguements
[then names account], then error#0x800ccc79

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Though everything goes thru fine, how do I stop this from popping up
after clicking send/recv.

Thanks,
Susan