What does the following mean in an error report?

Last error: 5.1.0 554 5.1.0 Sender denied

It looks like it means that a subscriber tried to send an email to the
listserv but was unable to. One of my subscribers sent an email to the
list and it was received in normal fashion, but she did not receive
it, herself, nor has she been receiving any of the listserv's posts.
Her address showed up on the error report with the notation above.

Thanks,
Pat

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