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?