By default, system-generated and notification messages sent to external recipients are generated by a user with an e-mail address postmaster@>. By default there is no mailbox-enabled user however in your Exchange organization with that email address. You can choose to give a mailbox-enabled user that default postmaster address manually or you can change the postmaster address by using the Set-TransportServer cmdlet as can be seen in the example below where I'm changing the external postmaster address to match [email protected].
[PS] C:\>Get-TransportServer | Fl Name,ExternalPostMasterAddress
Name: DC
ExternalPostmasterAddress :
[PS] C:\>Get-TransportServer | Set-TransportServer -ExternalPostmasterAddress [email protected]
[PS] C:\>Get-TransportServer | Fl Name,ExternalPostMasterAddress
Name: DC
ExternalPostmasterAddress : [email protected]
[PS] C:\>