Email - Load balancing - DNS

Which version of Exchange server you are using?

Likely what you'll do is this.

1) Setup your DNS round robin IPs. This is easy, if you're using a sensible DNS like named, just associated several IPs with an A record.

So something like this... for 3x IPs...

smtp IN  A 10.0.0.1
smtp IN  A 10.0.0.2
smtp IN  A 10.0.0.3

@  IN  MX 10  smtp

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This means the MX record for the origin (domain name) will rotate between the 3x IPs for smtp.

Again, if you use a sensible DNS system, this is all that's required.

2) Now you setup your scanners on all the IPs for the smtp host.

3) If one of the smtp host IPs is down, then the server doing the SMTP message submission will go into backoff mode + attempt a resend in the backup period.

Note: Generally, a better approach for MX records is do this...

mx1 IN  A 10.0.0.1
mx2 IN  A 10.0.0.2
mx3 IN  A 10.0.0.3

@  IN  MX 10  mx1
@  IN  MX 10  mx2
@  IN  MX 10  mx3

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In this case if one of the IPs fails to respond, the next MX record is tried, so you'll never hit a resubmission waiting for a backoff period.

This means each MX IP will respond in somewhat of a round robin fashion, although more loosely than a DNS round robin setup.

Note: Best to open a completely different question about how to setup DNS failover, as there really isn't such a thing.

What you're likely asking about is IP failover, so when to take an IP out of round robin rotation when the IP is dead.

The simple approach.

1) Run low TTLs 1-3 minutes.

2) If one IP stops answering (via a ping or DNS lookup or whatever)... then...

3) Remove this IP out of your zone file.

4) Recompile your zone file + push the new zone file to all your DNS instances.

This may sound complex + in practice, it's very easy to get this working... and there are many considerations if any other services are running besides DNS + SMTP submission.

For example if databases are running on these IPs, then the entire failover approach must change dramatically.

thanks for the info David, and sorry for the late reply.
I'm not sure what backoff mode is. We use Windows 2012 R2  DNS, and Exchange 2016.

Can you modify TTL's in Windows DNS?