Currently we use a hosting company for our Exchange services. We are considering a switch to Google apps, but that is in a pilot program at the present time.
Our Exchange provider and Google Apps both have the ability to send e-mail that has been addressed to unknown users to another SMTP server.
I'm looking for advice on how I could setup an SMTP server that would allow me to setup the following:
Allow me to enter a list, or perhaps create a user account for any past employees. Any mail messages sent to the past employee would have a custom response sent back to the sender explaining that this person is no longer at the company and they should re-send their e-mail to person X that is now handling the past employees work.
Send a NDR bounce message for any other user that is not defined in the system.
We are primarily a Windows shop, but I don't have a problem in setting up a Linux solution to handle this since it will be a fairly low maintenance system. We don't have that many employees come and go :-)
Also, if we do switch to Google Apps then I would also make this system an outgoing SMTP server to handle sending our outgoing mail from our web apps since Google limits each user account to 2,000 messages a day.
EDIT: I should explain this a little more. Right now, the company that we use for Exchange hosting does not send NDR's. They claim this is to prevent spammers from sending spam to their servers to fake accounts with real, but spoofed, e-mail address in the return path. Thus delivering the spam message to the intended recipient through the NDR mechanism.
By setting up a separate SMTP server that I would control and they would deliver all messages to that did not match a current account, then I can send NDR messages back to the sender.
We have had a problem in the past where someone sent an e-mail to us but had a typo in the address. However, since our provider does not send NDR's back they though the message was delivered, but we never know about the message.