We’re not going to be able to give you a solution based on this information…but we can get you searching in the right direction. This doesn’t match the description of how spam is handled on Virtualmin Professional systems, by default, so I’ll assume you’re just dealing with some sort of manually installed configuration. (If I’m wrong, and this is a Virtualmin Professional system, let me know…we can get much more specific and concise in how to fix your problems.)
Let’s get on with the mail service educatin’:
Postfix doesn’t, under any reasonable circumstance, modify the subject (I assume we’re talking the subject line–Postfix does modify message headers, but you wouldn’t see that unless you went looking for it). So, you’re looking for information about whatever spam filter is in use on your system. Pretty much all of them work with Postfix, and there are dozens or hundreds of possibilities out there, so it’s impossible to guess which particular product is munging your message subject line.
But, you can probably find out (or begin your search) by checking with postfix. First up, see if you’re delivering via procmail or using the built-in postfix delivery agent:
[[joe@delilah ~]]$ postconf mailbox_command
If Postfix is delivering, the result will be empty (“mailbox_command =”), but if procmail is delivering, you’ll have something like this:
mailbox_command = /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME
There are other delivery agents, of course, but procmail is the most popular, and the one built in to Postfix is also good and a popular delivery option. If you have something else in that field…you need to chase up docs on whatever that MDA happens to be.
If postfix is your delivery agent, then spam filtering will probably be happening via the filter feature (there’s also an open source spam proxy, which is a pretty stupid way to address the spam problem, but it has good marketing for an Open Source project, so it is pretty popular–I’m not sure how’d you’d spot that one since it happens before postfix enters the picture). So you can look at:
This will probably be amavisd-new, MailScanner, or direct use of SpamAssassin or some other spam filtering product.
Or, if procmail is the MDA, you’ll look at /etc/procmailrc to see where mail goes after it reaches procmail. You may have to chase through individual user .procmailrc files, as well, depending on how things are configured. procmail is a programming language and a mail delivery agent and mail processor, so it can be confusing to see for the first time–but if you see “spamassassin” or “spamc”, then you’ll know you need to start working on the SpamAssassin configuration. Other filtering products are obviously called some other way.
In summary: You won’t be altering any Postfix configuration to achieve your goal–this will happen in the spam filtering product, whatever that may be.
If you’re still not sure what to do with this information, bring it back to this thread and I’ll try to guide you further. You might be most successful taking it up with the support forum of whatever the spam filtering product is, though, particularly if it’s anything other than SpamAssassin (that’s the only one I’m really familiar with).