I seldom DELETE a userid based upon a simple bounce, unless it is from a
VM host (most of the time "auto-delete" will catch real "userid
unknown").
 
It seems that many bogus "userid unknown" messages are temporary because
of some problem in the other host authorization system.
 
Nevertheless, I frequently MAIL QUIET SET list NOM NOF FOR userid@host,
which preserves subscription info with a followup to the (postmaster or
root or system) at host.  Depending on the exact nature of the bounce,
I'll send a "nastygram" to a higher level e.g., if something from
host.subdomain.domain.top bounces, I'll try [log in to unmask]
(I usually use DiG subdomain.domain.top ANY to do a reality check,
followed by a telnet to that host port 25 VRFY on postmaster, root, or
system).
 
If I can't find those email userid/hosts, I'll go to the rs.internic.net
and do a WHOIS DOM domain and hope that I can find a userid not
associated with the errant host.  Sometimes I'll do a
DiG subdomain.domain.top SOA for other useful addreses.  And finally, if
all else fails (in finding an administrative address for certain
bounces), I'll do the DiG on one of the other authoritative name
servers.
 
Most of the time, it isn't that messy, but I have employed all of
these scenarios at one time or other.  (Thanks Melvin!)
 
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