I would be very careful with this for several reasons:
1. There is a very popular NO POSTMASTER antispam database, "postmaster.rfc-ignorant.org" which many now poll and, if the domain is not found to have a email@example.com address, will block or weight their spam according to the results returned.
2. YAHOO! demands a POSTMASTER@ e-mail address for every domain and, if someone complains about spam from a domain, actually begins checking all e-mail arriving from the domain for postmaster@
3. When setting up feedback loops, many of the domains now check, and verify, that a postmaster@ e-mail address exists for the domains for which feedback loops have been setup.
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