If I update my existing MX records, will I stop receiving mails for 72 hours

Question

I used the Google's "Check MX" application to check the functioning our website's MX records. It showed a critical error:

Status: ERR
aspmx.l.google.com MUST be the first MX record.

Your setup  
10  alt1.aspmx.l.google.com
20  alt2.aspmx.l.google.com
30  aspmx2.googlemail.com
30  aspmx3.googlemail.com
30  aspmx4.googlemail.com
30  aspmx5.googlemail.com

Recommended setup
1   ASPMX.L.GOOGLE.COM.
5   ALT1.ASPMX.L.GOOGLE.COM.
5   ALT2.ASPMX.L.GOOGLE.COM.
10  ALT3.ASPMX.L.GOOGLE.COM.
10  ALT4.ASPMX.L.GOOGLE.COM.

Right now we are receiving emails and are able to send them as well.

My question is if we go ahead and update the MX records, will we stop receiving emails till the time of DNS propagation. Or will the transition be seamless since we already have a working DNS setting?

Answer 1

The transition should be seamless.

In this case, where the email is working OK in its current (albeit slightly wrong) state and the correction will still send emails to the very same server then there would not seem to be anything to worry about.

To speed up the propagation you can reduce the Time To Live (TTL) on the DNS record to something like 300 seconds (5 minutes) a few days before you make the change, and change it back again afterwards. This is the length of time that DNS servers should cache the value. If the cache period is long then it will obviously take longer for the caches to expire and the records to propagate.

The DNS isn't "switched off" when it is changed, as you seem to fear. The DNS suddenly switches from the old to the new setting, except that this switch occurs at different times on the different DNS servers, somewhat dependent on the TTL.

The only time when this can be a little "tricky" is when you are changing to an entirely different server. What can happen is that whilst the DNS propagates some emails are sent to the old server and some to the new server. Although no emails should actually be lost. Again, reducing the TTL prior to making this change can reduce this problem.