What is the difference between MX priority 1,2,3,4 and 10,20,30,40?

There is no difference really.

The MX numbers describe in which order the email servers are contacted when trying to deliver an email message. The server with the lowest MX number has the highest priority. Email sent to your domain will therefore be sent to the server with the lowest MX value. If the server with the highest priority (lowest MX value) becomes unavailable for any reason the next highest mail server will be used. If that one is also not available the next highest and so on. If two mail servers have the same MX priority one will be picked randomly by the sending server.

It’s a relative number, so you don’t need an MX priority of 1 for your primary mail server but only have to make sure that it has the lowest MX value.

So these three versions all have the same effect:

1 aspmx.l.google.com.
2 alt1.aspmx.l.google.com
2 alt2.aspmx.l.google.com
3 aspmx2.googlemail.com
3 aspmx3.googlemail.com

1 aspmx.l.google.com.
5 alt1.aspmx.l.google.com
5 alt2.aspmx.l.google.com
10 aspmx2.googlemail.com
10 aspmx3.googlemail.com

10 aspmx.l.google.com.
20 alt1.aspmx.l.google.com
20 alt2.aspmx.l.google.com
30 aspmx2.googlemail.com
30 aspmx3.googlemail.com

Administrators like to use values like 10,20,30 instead of 1,2,3 because you can just insert another email server before or in between without having to change the existing values.
If you only have one single mail server you can choose any value between 0 and 65535 (the maximum possible MX value since it’s a 16-bit value) as MX record.

What is the difference between MX priority 1,2,3,4 and 10,20,30,40?
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