I'm currently using postfix-2.11.3, and I am doing a lot of message processing through a milter. This processing takes place before the client is notified that the message is accepted, and it sometimes involves enough work that it delays the client's receipt of the initial SMTP 250 2.0.0 Ok: queued as xxxxxxxxxxx message.
During large email blasts to my server, this milter processing can cause a backlog, and in some cases, the client connections time out while waiting for that initial 250 ... message.
My question is this: if I rewrite my milter as a postfix after-queue filter with no before-queue processing, will clients indeed get the initial 250 messages right away, with perhaps subsequent SMTP messages coming later? Or will the 250 message still be deferred until after postfix completes the after-queue filtering?
And is it possible for an initial 250 message to be received by the client with a subsequent 4xx or 5xx message received and processed later by that same client, in case the after-queue filter decides to subsequently reject the message?
I know I could test this by writing an after-queue filter. However, my email server is busy, and I don't have a test server available, and so I'd like to know in advance whether an after-queue filter can behave in this manner.
Thank you for any wisdom you could share about this.
I managed to set up a postfix instance on a test machine, and I was able to install a dummy after-queue filter. This allowed me to figure out the answer to my question. It turns out that postfix indeed sends the 250 2.0.0 Ok: queued as xxxxxxxxxxx message before the after-queue filter completes.
This means that I can indeed move my slower milter processing to the after-queue filter in order give senders a quicker SMTP response.