SMTP server requirements

Could someone help answer a few questions regarding the networking / DNS requirements for an SMTP server? Assume the server is setup already.

1. Is a MX record required for SMTP?
2. TCP port 25 open to the server?
3. Any other DN开发者_开发问答S records?

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I think this depends on what you mean with 'required'.

Technically you need an application listening to a TCP port. No DNS is necessary as others can access the box with the IP address.

However when you think about 'public' SMTP server that can interact with arbitrary SMTP clients so you can receive mails from gmail etc, you need: - server listens on TCP port 25 - either an MX record or SMTP server is on the machine where the A/AAAA record points to