We're using a 3rd-party service provider to send transactional email. I recently noticed increased failure rates for a given receiving domain.
The sends fail with the error "498 No MX for example.com".
The sends are retried after a given delay and then usually succeed after a couple retries. But sometimes, they exceed the retry limit and are dropped permanently.
I contacted the support of the provider and they told me that this is due to the receiving domain declaring MX from different providers.
$ dig mx example.com ;; ANSWER SECTION: example.com. 859 IN MX 25 mail05.example.com. example.com. 859 IN MX 20 mail11.example.net.
They are referring to the fact that one MX is using
example.com and the other is using
example.net and that is apparently bad practice and can lead to the error described above.
This is the first time I'm hearing something like that and I would instantly call BS on it, but I thought I'd give them the benefit of the doubt and hear what others have to say on the topic.