SpamAssassin is a powerful mail filter, which uses text analysis to determine the probability of a mail message being SPAM. Some of the factors for this decision are the presence of HTML parts in the mail, phrases common for SPAM (“drastically reduced”, “save big money”, etc.), excessive HTML comment tags (often used to ‘hide’ common phrases from mail filters), invisible text (using the same foreground color as the background color) and many others. Each occurrence of something suspicious raises the probability of the message being SPAM (the SPAM level). You can configure the SPAM level above which the mail messages will be filtered.
The levels manage how strictly SpamAssassin reviews each message to consider whether it is SPAM or not.
Setting the SPAM level to a low value (1/10) will probably cause legitimate mail to be filtered. Setting it to a high value (10/10) may let some SPAM into your mailbox. The optimal level depends on the contents of the mail you receive – you will have to adjust it until you find the lowest level possible, at which no legitimate mail is filtered (you might need to check regularly your Junk mail folder to see if you have legitimate messages filtered).
Statistics show that 5/10 is generally the optimal spam level. It usually does not result in filtering legitimate email, while it is still efficient against spam. However, we do recommend that you check your Junk mail folder regularly, in case there are filtered legitimate messages.
Enabling SpamAssassin globally will make it process any message, sent to your domain, while enabling it per user basis will make it scan only the messages, destined for the respective mailbox.