The default score for SpamAssassin is 5.0.  If an email scores higher than this number, then it is considered spam, unless there are modifiers (such as the FROM: address is in your whitelist).

Section 8 of the MAIA documentation details how to change this number.  You will be using the Webmin File Manager to edit /var/www/html/maia/config.php and changing the number '5' to be whatever score works best for your organization.  I have found 5 to be a bit aggressive, and 6.5 or 7.0 works a bit better.

Exchange 2003 SP2 and Exchange 2007 have the IMF built-in.  I generally set the IMF to a value of 9, and use the reject setting.  This helps a bit, without catching anything tooo obvious.  Note:  the IMF can cause weird problems with in-house scan-to-email devices, fax-to-email services, and VoIP systems that email your voicemail messages to you.  Make sure to create a seperate SMTP connector for just those services.

Typical Headers might look like the ones below.  Notice the scores applied for different tests (highlighted in maroon).  Unlike most commercial services, you know exactly what this spamfilter is thinking.  It is very easy to adjust these scores - you can use the Webmin File manager to edit /etc/mail/spamassassin/local.cf and adjust the scores.  An example is in the file already.

X-Spam-Flag: YES
X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on relay1
X-Spam-Level: **************************************************
X-Spam-Status: Yes, score=1006.7 required=5.0 tests=DCC_CHECK,DIGEST_MULTIPLE,
        GTUBE,NO_RECEIVED,NO_RELAYS,PYZOR_CHECK,RAZOR2_CF_RANGE_51_100,
        RAZOR2_CF_RANGE_E4_51_100,RAZOR2_CHECK autolearn=no version=3.2.5
X-Spam-Report:
        * -0.0 NO_RELAYS Informational: message was not relayed via SMTP
        * 1000 GTUBE BODY: Generic Test for Unsolicited Bulk Email
        *  1.5 RAZOR2_CF_RANGE_E4_51_100 Razor2 gives engine 4 confidence level
        *      above 50%
        *      [cf: 100]
        *  0.5 RAZOR2_CHECK Listed in Razor2 (http://razor.sf.net/)
        *  0.5 RAZOR2_CF_RANGE_51_100 Razor2 gives confidence level above 50%
        *      [cf: 100]
        *  2.8 PYZOR_CHECK Listed in Pyzor (http://pyzor.sf.net/)
        *  1.4 DCC_CHECK Listed in DCC (http://rhyolite.com/anti-spam/dcc/)
        *  0.0 DIGEST_MULTIPLE Message hits more than one network digest check
        * -0.0 NO_RECEIVED Informational: message has no Received headers