milter | Go package for writing milters

for a while now I am trying to install Spamassassin properly as root user with 'apt install spamassassin spamc'. The installation fails when it runs sa-compile producing following output:

I have followed this tutorial to configure DKIM and Postfix on Debian 7 wheezy. These instructions are pretty much a standard on the interwebz.

I am using Gmail to send and receive emails using my own domain. I followed this instructions to achieve that.

My problem

I can send and receive emails but I can't manage to pass the DKIM test (at least with Gmail). After searching and struggling for a while I have come to the conclusion that the reason of my woes is that my message is getting multiple DKIM signatures (see mail.log below). And this, according to the DKIM directives is enough for the DKIM to fail.

But after reading about on how to solve the multiple signatures problem I found that absolutely all of these solutions refer to having 'amavis' installed. Thing is...I don't have it installed!

In any case these solutions mention changing postfix configurations related to the milters in master.cnf and/or main.cfn. For example, adding this to the 'receive_override_options' (again, I don't have that variable since I don't have amavis installed) should solve the issue:

Is there any way that a milter under postfix can be told that a given client connection attempt failed specifically because of an authentication error?

Sometimes, my email server gets blasted with repeated SMTP authentication attempts from the same client, presumably in order to try to break through my security. I'm running a milter, and these attempts show up as a CONNECT followed by a CLOSE, with an error message in my postfix log that contains info like this:

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've created myself a mailserver with postfix, dovecot, spamasasin.. Was looking in the logs today and found a lot of this