My spam traffic has decreased to minimum after I've inserted following lines in /etc/postfix/main.cf file (Postfix main configuration file). These settings made my SMTP server to spam resist. This method use DNS queries, what mean very fast UDP communication and use less memory and CPU then any spamfilter. If your hardware is weak or an all-in-one server than use this really fast and effective method.
Before: I received 200-300 spam a day.
Now: I receive only 1-2 unsolicited email a day.
Here is a simple UTF-8 mail sender function. This function also encode subject and plain-text message to UTF-8. If you need HTML mail sender, change the code in line 4 from text/plain to text/html, but this function is usable the most cases without any modification. I've used this function for webform and other web based mail notification.
