5xx responses and sender lists (was facebook...)

Jay Hennigan jay at west.net
Fri Mar 5 16:57:59 UTC 2010


On 3/5/10 7:08 AM, David E. Smith wrote:

> As long as we're going off-topic, might as well go all the way :V
> 
> How long should a sender (say, Facebook) retain a database of 5xx SMTP
> responses? Just because jimbob at school.edu doesn't exist today, doesn't mean
> that James Robert Jones won't enroll in the fall and get jimbob@ as his
> school-provided email address.

They really don't need to retain it at all, other than perhaps to count
a few messages to determine to remove the address.

A 5xx response is a permanent failure.  "The specific message you are
sending to this address *right now* will never be delivered, don't keep
trying to send this specific message."   This usually means that the
address does not exist, is administratively prohibited from receiving
messages, the MTA is blocking the sender, etc.  It is possible but
unlikely that a future message from the same sender to the same
recipient will succeed.  Some mail systems with "dirty word" filters may
return 5xx to a specific message and allow another with different
content.  These aren't all that common.  Things like a user going over
quota or a temporary failure should not return 5xx but 4xx.

Facebook (and legitimate senders of periodic subscription emails) should
remove that address from their subscriber lists after receiving 5xx
responses.  Some subscription services will delete an address only after
a series of 5XX rejects (three in a row for example) rather than just
one to guard against a fluke erroneous response.

If in the future jimbob@ gets assigned that address and chooses to
subscribe, then it can be re-added.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV




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