make local enqueing always be fast?

[l...@squareup.com]

Can Postfix be configured such that, when a message is queued from a process on the same machine, then the message will be accepted instantly, without blocking on any traffic over the public Internet?

My scenario is that we're sending emails to our users for a variety of reasons, including password resets, two-factor authentication codes, and new-device notifications. These emails get queued by automated systems. Due to the use of an RPC/SOA style of architecture, end users can end up waiting while the message gets queued synchronously.

We're seeing that most of the time--well over 99%--Postfix accepts messages instantaneously when delivered locally over SMTP. However, some small fraction of the time, Postfix blocks, sometimes for more than ten seconds, before eventually accepting the mail. The machines in question are not loaded, much less swapping.

My first question is what is happening? Is there perhaps some DNS lookup that is slowing things down?

My second question is can we configure that off? I see configuration options to turn off DNS lookups in the Postfix SMTP *client*. However, I see no obvious configuration options to do the same thing in the smtpd server. This is assuming it's DNS at all, but what else could smtpd be doing?

Any input would be welcome. If Postfix can't be configured this way, we will likely end up putting a separate queue of our own in front of Postfix. That seems architecturally silly, though, given that Postfix has a superb email queue built into it.

Lex Spoon