Strange sender denied NDR in postmaster mailbox

[gtstangman35]

Good Morning,

I am having an issue where I am getting about 6-8 NDR's in my postmaster mailbox from our Front end server with the following message:


The e-mail address could not be found. Perhaps the recipient moved to a different e-mail organization, or there was a mistake in the address. Check the address and try again.
<emailserver.myorg.com #5.1.0 smtp;554 5.1.0 Sender Denied>

These emails are to valid people in our organization, why are these NDR's created?

Thanks,
Wes H