As I stated before, I really like Christoph Haas’ ISPMail setup for Debian-based mailservers. I was quite fine without any server-side spam filtering solution until now, but somehow the spam amount in my inboxes increased more and more and I was looking for a decent and simple solution to filter out all that bullshit which is distracting me day after day.

I clearly wanted to go with Spamassassin (SA), as I made good experiences with it in the past and it’s more or less the standard spamfilter on linux based mailservers. The most common solutions to integrate SA into a Postfix based mailserver are the following:

• Using amavisd-new
• Using Postfixs content_filter

I don’t really like both of them. Amavis is quite heavy for the pure spam filtering purpose and the content filter checks both ingoing and outgoing mails by default which is obviously not in my interest. Amavis avoids checking outgoing mail just by checking if the sender domain is managed by the same system, but spammers can bypass this quite easily by faking the sender’s address to be the same as the recipient’s one (which is done quite often). There’s a discussion about this on the ISPMail page, so head there for more information. All this can be improved by using multiple Postfix instances and different ports (e.g. using 587/submission for authenticated clients and 25/smtp for normal SMTP traffic), but I want my mailserver to be as interoperable as possible without the need of any special setups on the client side.

So I was looking for another solution. I read some tutorials where people used procmail in user scripts to pass incoming mail to spamc before delivering it to the mailbox. I like this approach as the MTA isn’t involved into the spam filtering process, outgoing mail isn’t touched and you don’t need any complicated setups on the MTA side. All alias and transport definitions work fine and the final mail is checked right before being delivered to the user’s inbox.

First I thought about Sieve, which is already running through Dovecot’s Sieve implementation until I noticed that Sieve is not able to call any external programs (correct me if I’m wrong). Then I had a look at spamc and Postfix’ master.cf. spamc is capable to pipe its output to another program and in the ISPMail setup, Postfix passes the mail directly to Dovecot’s deliver, so why not just let Spamassassin check the mail right before it’s getting passed to Dovecot? I gave it a try and seems to work fine. I still need some automation in training SA databases (might follow in a later post), but the plain SA checking is working reliably and mails can easily be filtered with Sieve afterwards.

So much for the backstory, let’s get our hands dirty. Note: I’m running Debian Lenny.

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