There are clearly benefits of using a subdomain for sending email to protection domain reputation, but is this always true? What about the extreme case, where a spam domain sends every email from a distinct subdomain?
1 Answer
For the recipient, identifiers stored in a domain name (your arbitrary choice) & identifiers stored in the mailbox name (also your arbitrary choice) are not that different, except in the former case you are causing some additional useless DNS traffic.
So what if you send every mail from a unique subdomain? You gain nothing, but you lose some nuance, and have some management overhead.
If you use more than X (where X is a small positive integer) subdomains to send mail, recipients reputation management will separately track fewer or none of your subdomains.
Worst case: The recipients just stops saving your individual subdomains in their database - all your domains will get treated the same. The most extreme thing that might happen if you exceed anti-DoS limits is that you prevent yourself from benefiting from the recipients willingness to treat your subdomains differently.
E.g.: A mishap in your accounting department will now more directly affect your invoices and newsletters. The size of the effect is generally not visible to you, different recipients will care much or less about domain names.
