Question: an up-stream server must not reject unverified recipients
when the down-stream server is unavailable.
Answer: let the address verification cache (address_verify_map) in
the up-stream server work for you.
Once an address is stored in the address verification cache (this
requires that the down-stream mail server is sometimes available)
the address remains cached for 31 days.
Also, when the down-stream mail server is available, the up-stream
server will "refresh" a cached address that is older than 15 days.
This refresh happens only when an address receives mail; an inactive
address will be dropped from the cache when it is older than 31 days.
If you are really worried that a "new" or "inactive" user isn't
cached, you could use a small program that runs on the down-stream
server and that sends "rcpt to" commands to the up-stream mail
server. That script has to respect the smtpd_recipient_limit setting
(default: 1000).
Wietse
