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I'm trying to secure my MTA against relaying spam, and am trying to understand what's involved.

For example: Say I send an email using an Yahoo account to a Gmail account. When Yahoo is relaying the email to Gmail, does the relay take place without authentication? What do you need as the server to be able to successfully relay emails to another smtp server?

How do I set up an MTA to prevent spam relay?

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    What alphamikevictor said below. I also wanted to add that if the mail servers are properly configured (to prevent spam relay), then in your example above Yahoo will only accept mail that is authenticated or from its proper network, and Gmail will only accept mail that's for Gmail.
    – Katherine Villyard
    Commented Apr 17, 2015 at 14:50
  • My question really is, how to set up an MTA to prevent spam relay
    – user1720897
    Commented Apr 17, 2015 at 14:53
  • Ah, that's not exactly what you asked.
    – Katherine Villyard
    Commented Apr 17, 2015 at 15:09
  • 1
    It would help if you told us what MTA you were using.
    – Katherine Villyard
    Commented Apr 17, 2015 at 15:30
  • @KatherineVillyard Exim
    – user1720897
    Commented Apr 18, 2015 at 0:49

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Basically they perform a query for MX record of the domain, following your question, for gmail.com:

alphamikevictor@ges01:~ > nslookup -query=mx gmail.com
Server:         194.179.1.100
Address:        194.179.1.100#53

Non-authoritative answer:
gmail.com       mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
gmail.com       mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail.com       mail exchanger = 5 gmail-smtp-in.l.google.com.
gmail.com       mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com       mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.

Authoritative answers can be found from:
alt3.gmail-smtp-in.l.google.com internet address = 64.233.189.27
alt4.gmail-smtp-in.l.google.com internet address = 173.194.72.27
alt1.gmail-smtp-in.l.google.com internet address = 64.233.165.27
alt2.gmail-smtp-in.l.google.com internet address = 74.125.200.26

For each authorized SMTP server you have a weigh so Yahoo's MTA will try to deliver to gmail address using this servers:

  1. gmail-smtp-in.l.google.com
  2. alt1.gmail-smtp-in.l.google.com
  3. alt2.gmail-smtp-in.l.google.com
  4. alt3.gmail-smtp-in.l.google.com
  5. alt4.gmail-smtp-in.l.google.com

You can read more about MX record at WikiPedia http://en.wikipedia.org/wiki/MX_record

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  • So, Yahoo's MTA will be able to send the mail to GMail's MTA without any authentication? Which means that if I set up an MTA, I would be able to send mails to another domain without authentication, or in other words, spam?
    – user1720897
    Commented Apr 17, 2015 at 14:51
  • As far as gmail MTA's are getting the mail for themselves there is no need to authenticate (normally). You need authentication when you use Gmail MTA to send mail to another domain otherwise anyone would use Gmail MTAs to send mails all over the world. Or look with another point of view: you as an external user, how do you authenticate to other MTA system which has no relationship with you?
    – alphamikevictor
    Commented Apr 17, 2015 at 15:09
  • @user1720897 Yes, if a mail server is authoritative for example.com, you end up accepting spam for example.com. Alas. That's a whole other topic. The point is more that if your mail server is authoritative for example.com, you don't send spam to other domains.
    – Katherine Villyard
    Commented Apr 17, 2015 at 15:15

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