Interpreting NDRs

I believe I have relaying properly prohibited on my Exchange 5.5 SP4 (W2000) server because when I test it via telnet, it denies relaying. However, I am still receiving several NDRs that make me think somehow the relays are still getting through. Examples:

550 Requested action not taken:user account inactive
550 Relaying is prohibited
550 <***@***.com>… User unknown
553 VS10-RT Possible forgery or deactivated due to abuse

TIA!!!