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How Does a Mail Server Respond to Fake Email Addresses?

During a security assessment, I found that I could connect to the SMTP gateway using Telnet. I tried sending mail from a fake domain, but it was detected as a mail relay and stopped. When I sent messages to fake employees inside the organization's domain, however, the mails were accepted. Can this be termed as a mail relay vulnerability? Can this be exploited for purposes other than social engineering? Most importantly, what is the best possible resolution?

What you describe is actually a very common situation and is not a cause for alarm. You can Telnet to most mail servers on TCP port 25 and send messages to the organization that uses the particular server. But, you should not be able to send email to other organizations. If you could, a spammer would find that mail server and use it to relay spam.

So, what actions should the mail server take if the destination email address is fake?

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posted by daniel.toma, on 25 May 2008
Most server perform a catch-all action for non-existent recipients and deliver the messages to the postmaster account or the server administrator's account. Some of them send a NDR message stating that the recipient address does not exist. Others respond with a permanent error condition stating that the recipient address does not exits during the SMTP communication phase.
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