Erik Aigner asked:
I was wondering how to distinguish inbound from outbound SMTP transactions.
Normally this would be a straightforward question, however my SMTP server has to handle a special case:
- Other domains might point to my MX domain (mail address aliases)
What outbound criteria should I check for?
Is it as simple as checking if the transaction is authenticated (via the AUTH command), or do any other rules apply (derived from the FROM/RCPT addresses)?
I guess alias addresses can be detected by resolving their MX?
My answer:
If the sender is in your domain, it’s outbound.
If the recipient is in your domain, it’s inbound.
If neither are in your domain, it’s relay, and drop it like a hot potato.
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