Lately, I received emails from a customer something like 12 hours later...

So I checked closer and could see that the email was from hotmail.com.

Great.

On the server, I edited my whitelist to see what was going on:

vim /etc/postgrey/whitelist_clients.local

Searching the list I could see hotmail.com, so why is it that it gets bounced with a 450 by Postgrey if it is whitelisted?

Looking closer at the logs left behind by Postgrey, I found this line:

Apr 23 06:33:34 m2osw postgrey[1742]: action=greylist,
      reason=new,
      client_name=<secret>.outlook.com,
      client_address=40.92.10.103,
      sender=xxx@hotmail.com,
      recipient=xxx@diverserealty.team

As we can see, the sender has an email address that says "@hotmail.com".

Only, that's nice, but the Postgrey code actually makes use of the client_name parameter and there the domain is clearly outlook.com.

Here we go! I had to also whitelist outlook.com. The next very surprising things, though, is the fact that Postgrey would forget that outlook.com is fine. I clearly had the default auto-whitelist turned on... Oh well, anyway, after adding that extra host, it works as expected.