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We have several domains all pointing their MX records at mail.ourdomain.com, an internal mail server.

We are looking to outsource our email to a new supplier, who would like us to use mail.newsupplier.com; their mail server.

We'd rather not change all of the domain names to point to that MX record; several aren't in our control, and it would mean attempting to get many parties to change their MX records at the same time, which seems problematic.

Simpler would be to repoint mail.ourdomain.com at the IP for the new supplier. The problem is that our supplier isn't able to guarantee that IP will be fixed.

My question is, therefore: is changing mail.ourdomain.com to CNAME to mail.newsupplier.com an acceptable solution?

(For the record, only the email is moving, so we'd want to leave www.ourdomain.com and everythingelse.ourdomain.com unchanged.)

I've found several messages warning of the dangers of CNAMES in MX records, but I can't quite find someone talking about this particular setup, so any advice will be greatfully received.

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According to RFC 1123, the MX record cannot point to a CNAME. If I were in your situation, I would setup mail.ourdomain.com as an A record pointing to the new suppliers IP address and then quickly work on changing all MX records over to the correct data. Then address why changing MX records is so difficult in your organization.

That being said, most mail servers will still submit mail to a CNAME; however, you can't be guaranteed of it.

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    I would stick to the RFC: No you can't. – Benoit Nov 4 '09 at 18:26
  • Could you please clarify the RFC reference? Looking at RFC 1123, I can only see it saying that the domain names used in MAIL and RCPT commands cannot be a CNAME (in section 5.2.2). I cannot find anything in it about an MX record pointing to a CNAME. I also cannot find any particular reference to CNAMEs in RFCs 973 or 974, which defined MX records. – Dolda2000 Jun 1 '15 at 14:20
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    RFC 2181 §10.3 is clearer on the matter: "The domain name used as the value of a NS resource record, or part of the value of a MX resource record must not be an alias." and "This domain name must have as its value one or more address records." – mr.spuratic Jul 26 '17 at 10:04
  • tools.ietf.org/html/rfc5321#section-5.1 says the same thing too. paragraph 4: When a domain name associated with an MX RR is looked up and the associated data field obtained, the data field of that response MUST contain a domain name. That domain name, when queried, MUST return at least one address record (e.g., A or AAAA RR) that gives the IP address of the SMTP server to which the message should be directed. – Jasen May 6 '18 at 23:32
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There are companies that provide Backup SMTP Service aka email store and forward. Perhaps you could use one of those companies to wash the IP for you.

If they can't be the permanent middleman maybe just having them as a secondary MX record will help with the transition process so you don't have undeliverables bouncing around.

If you are currently speaking of one of those sorts of companies maybe you could second source it and have them forward back to the first source.

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