On 12/09/2017 22:36, Brandon Long via mailop wrote:
So, looking at our code, for our daily send limits for consumers, we
do use a 550 response.
Thinking about this, it makes sense. It's not a short term temporary
condition, it's something that could take hours to resolve, and having
your mail sit in your outbox for who knows how long retrying, and
maybe the user doesn't notice and shuts down the computer or whatever,
it never goes out.
This changes if you have another MSA submitting to yours. We have this
here when we have customers running our mail server software on their
networks and send through our relay service. (They can't have their
local mail server send direct because they don't want the hassle of
handling all the stuff involved with doing that). We charge for this
service based on usage, so they have a monthly allowance.
So, they may have 100 people sending mail to their MSA which then sends
it to ours.
If they reach their send allowance, and we reject with a 4xx response
then their MSA queues them up. Then, the local mail admin can contact us
and say 'oops, we need to increase our sending allowance', we charge
them a few quid more, and their MSA then sends all the messages with no
more user interaction.
If we reject with a 5xx response, then their MSA sends NDRs to all the
users, who then have to re-send all the failed messages. That may be a
lot of messages depending on how they have their MSA set up and how busy
they are. The users won't know what is going on (you can't expect them
to read the NDR), so they'll waste their time trying to resend when it's
just going to fail again. So, the mail admin gets hassled by users, and
has to get in touch with each of them and walk them through how to read
NDRs, how to check whether messages were sent or not, and how to re-send
messages, the user may even have to rewrite messages from scratch
depending on how they have things set up.
So, we use 4xx when rejecting due to send limits, because it's less
hassle for the users (and admins) in this situation.
To be honest, whether an MSA rejects a message with a 4xx or a 5xx
response, I'd expect the MUA to alert a user that the message can't be
sent. I wouldn't expect an MUA to queue up messages which failed with a
4xx response because that's not what the user would expect to happen.
But, maybe that's a bit optimistic of me.
So, I'd say reject with a 4xx response because MUAs should tell the user
immediately and MSAs should automatically resend the messages when the
problem is resolved.
Paul
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