We are currently attempting to move away from using a "local" mail(exchange) server to an cloud based offering for all our automated emails. The problem is that we send and receive thousands for emails a day and its uptime is quite critical so the business do not want to put all their eggs in one basket, so if we would like to use a cloud based offering(mailgun) they would like a backup if this goes down. So my question is:
Would it be possible to set multpile A, TXT and CNAME records to multiple IP address so if one mail server goes down we can automatically start sending emails from the fallover(without them being blocked doing a reverse DNS lookup)? I know we will still need to adjust the MX record for incoming emails but that is acceptable to not receive emails for a short(1-2 hours) of time.
Does this make sense?
Best Answer
You can't (easily) have machines in different locations with the same IP address. You don't need an 'A' record to send email from a server. You cannot implement failover by changing DNS records.
The important stuff is to have any server which sends emails listed in your SPF TXT records (if you actually have a published SPF).
If you are talking about a MTA gateway machine, (SMTP messages originate on other machines and are all routed through this server before accessing the rest of the world) then that's a different matter - you can influence this using DNS but not by waiting for an outage to change the records. SMTP is designed to accomodate such scenario by publising multiple DNS MX records with different priorities, Clients should attempt to connect to each server in the zone ordered by the MX priority - if it fails to connect, it should then try the next. in practice most MUAs won't bother looking further than the primary MX - but if the client is an MTA, then IME, these all behave correctly. You haven't told us how your outgoing emails get to the MTA - so it's difficult to advise whether this will work in practice.
Sorry, but you clearly have no understanding of DNS and little understanding of SMTP. You need to do a lot more reading than just looking at the answers here.