Understanding the MX records on ns1.box.example.com

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I noticed recently that MIAB appears to set MX records for basically every subdomain to ., the external DNS page indicates this is “Recommended. Prevents use of this domain name for incoming mail.”

But I’m a bit confused at this idea. The lack of an MX record prevents the use of the domain for incoming mail, does it not? Adding an MX record and setting it to the dns root seems unnecessary, and I’m having a hard time finding documentation that would suggest this behavior is common or typical. Does anyone have anything they can point to for why this pattern is preferable?

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See e.g. the first answer here Basically, it’s considered better to explicitly indicate a specific subdomain does not handle email.

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Oh fascinating, that’s super helpful. I didn’t know there was a fallback behavior about contacting hosts via their address records. I want to say it seems like that fallback should be eliminated in the modern internet, but then I’m sure some other weird specific things would break in unexpected ways. This definitely answers my question though, thank you!