I had websites state that the address was invalid because I didn't have an MX record, despite it not being an RFC requirment.
Also, a lifetime ago, had Hurricane Electric (https://he.net) refuse to accept a sign-up because... I only had one MX record. I added an additional record pointing the same IP, but they weren't happy.. So I allocated an additional ipv4 address to the same server, just to satisfy them.
You really should have an MX record though. Fallback to A is deprecated and really should have been removed by now.
HE provides professional network services and probably, perhaps even contractually, requires you to have two different ways to reach you. Everyone at that level has quick ways to get in contact with all their business partners, usually a phone number that's active 24/7.
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I had websites state that the address was invalid because I didn't have an MX record, despite it not being an RFC requirment.
Also, a lifetime ago, had Hurricane Electric (https://he.net) refuse to accept a sign-up because... I only had one MX record. I added an additional record pointing the same IP, but they weren't happy.. So I allocated an additional ipv4 address to the same server, just to satisfy them.
It's a losing battle.
You really should have an MX record though. Fallback to A is deprecated and really should have been removed by now.
HE provides professional network services and probably, perhaps even contractually, requires you to have two different ways to reach you. Everyone at that level has quick ways to get in contact with all their business partners, usually a phone number that's active 24/7.