From: Noel Jones (njonesmegan.vbhcs.org)
Date: Tue Jul 01 2003 - 18:24:35 CDT

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At 03:42 PM 7/1/03 -0700, Marcus Reid wrote:
>If the destination mail server is down, the incoming message
>will be deferred with a 450. This defeats the point of being
>a backup MX for domains. I'd like to be able to reject email
>to invalid recipients, but if the verification times out
>(after 9 seconds or whatever,) I would like to accept the mail
>into the queue instead of deferring it.

So if you're a backup MX for some domain, it seems you would only receive
mail for them when their server is down. Active recipient address
verification couldn't possibly work under those conditions.

After the cache of valid names is built up, you will accept mail for known
valid users even if the destination is down. Previously verified names
stay valid for a configurable period of time so you don't hammer the
destination server with verification requests.
address_verify_positive_expire_time = 31d

>Also, it seems bad to cache failed verifications. If an account
>is added to the destination server, the MX will still reject
>mail with a 550. Caching positives is good, because if the
>account is deleted later all that happens is a bounce is
>generated down the line.

This is configurable. Either don't cache negative queries or use a short
timeout, whatever fits your local setup best.
address_verify_negative_cache = yes
address_verify_negative_expire_time = 3d
address_verify_negative_refresh_time = 2h

--
Noel Jones

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