Random users getting bounceback messages

I have a client who keeps getting bounceback messages, as follows, from various senders. Yet, these same senders are able to successfully email her from time to time. Here are the messages:

[Return Code 550] sid: yrnHF1j00107gsyq01 :: Delivery to bethany@nve.com failed because the recipient doesn't exist.

[Return Code 550] sid: yGUL1j0015GyNsw01 :: Delivery to bethany@nve.com failed because the recipient doesn't exist.

I welcome all ideas on how to troubleshoot this.

Thanks,

Wayne