550 Mailbox unavailable or access denied.

A user received the above error message from three clients when trying to send email to a company called Sentry.

I sent all email addresses a test message with a delivery receipt requested. I received a delivery notification from all three emails and I received feedback from one of the email addresses. I am connected via exchange and the user is connected via POP3/SMTP. I copied the email address straight from her error message so I know she typed them in correctly.

Does anyone know why this is happening to her?

Thanks for your time.