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I've setup Postfix for email delivery on a client's server and made sure that our PTR records, SPF, and reverse DNS is all configured correctly.

As far as I can tell, we're doing everything correctly as listed in the "Bulk Senders Guidelines" published by Google, but it appears as though there are still a few messages that aren't being delivered.

We typically send out batches of a few hundred messages to about two hundred or so unique addresses. This month, two of the recipients contacted us to say that they didn't receive all our messages. For the first one, out of fourteen we sent to them, they only received ten.

I dug through the mail logs and it looks as though they have some sort of rate limiting going on. After five connections, they refuse any more for the next hour. I can see postfix re-attempting the connection every ten minutes or so, and it eventually succeeds in delivery the next five, then about an hour later, the final four. I suspect the last four are the ones that went missing, but I can't say for sure as I only have mail IDs to go on.

The second recipient is even more troubling as the logs are indicating that all three of the messages we sent were successfully received and there were no issues with delivery.

At this point, I have no idea what could be going wrong, so any hints would be much appreciated. I'm hoping to hear from the recipient's respective I.T. department to assist with trouble-shooting further, but in case they have no interest in assisting me with this issue, is there anything else I should be looking into?

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    If the other end is rate limiting, there's nothing you can do; they will have to change the limits or whitelist you. – Michael Hampton Jul 10 '15 at 17:46
  • I'm okay with being rate-limited, as there's no reason the mail has to be delivered in real time. I'm concerned that the log is showing that all fourteen messages are delivered (after the delay), but the recipient is saying that they aren't receiving them all. Are you suggesting that the rate-limiting might be responsible for the missing messages? – notanumber Jul 10 '15 at 18:25
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    If you actually delivered the messages and the other end accepted them, it's on the other end. – Michael Hampton Jul 10 '15 at 18:26
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If your mail server has successfully passed the messages to another server and got a 200 status back then it's the responsibility of that other server to either pass them on further or send a non-delivery notification to the envelope sender address stating a reason for the non-delivery. That's the SMTP contract. If that isn't satisfied, the operators of the server that produced the 200 status need to investigate.

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