About AS's behavior, please let me describe some infomation to you.
 
What kind email we need to do "Anti Spam" when email server loacted in the LAN site(same as your topoloy).
 
    Case A. Iwiz(my name) send email to Pratchaya, the email will be send from our email server  to your email server to your PC.
  
    Between our email server and your email server, your vigor 5510 should check SMTP mail from WAN to LAN(we should enable SMTP in Anti Spam and a firewall rule for disable SMTP from lan to WAN), so Anti spam is used for this kind email.(someone in the internet send email to our email server which located in vigor 5510 lan site)
   
    Case B.  Pratchaya send email to Iwiz, email from your PC to your email server to our email server(draytek) to me.
       Your Vigor5510 don't need to check your email, because your are trused and u loacted in the lan site.
       So SMTP from LAN to WAN(from your email server to our email server) don't need to check.
 
 
What kind email we need to do "Anti Spam" when email server loacted in the Internet or WAN site (same as corecasys).  
    
   Case A. When Mr.Gunlapode send email to me, mail is from his pc to email server(ex. hotmail) then to our email server to me.
                  It's POP3 from LAN to WAN.  We don't need to check email from LAN to WAN.
 
   Case B. When Iwiz send email to Mr.Gunlapode, email will be send from Iwiz PC to dreaytek email server to hotmail server to Mr.Gunlapode's PC.
                 It's POP 3 from WAN to LAN. So we need to enable POP3 from WAN to LAN.
 
After login your Vigor 5510, we find you only enable POP3, so we changed it to SMTP.
And now we keep to check the syslog from your Vigor 5510, it did work fine now.
We checked the syslog, and I try to describe some to you.
After you can check the syslog by yourself to see if anti spam work or not.
All email incoming to your email server will be showed in syslog, and you can see if it is a spam or not.
If it is a spam email, you will find the tag in the subject.
 

 
134Nov 19 14:40:445510[AS][Tag][Category=SPAM, RefID=0001.0A090202.4923BA4E.0064,ss=4,sh,fgs=0][@S:R=13:1, 122.168.59.85:11271->61.7.147.149:25][SMTP][Sender=heehawedmy14@bcwinstitute.com, Receiver=poomsak@ruamkij.com, Subject=Price for 100mg x 60 pills $2.17 per pill]

 
This email is from [Sender=heehawedmy14@bcwinstitute.com, to  Receiver=poomsak@ruamkij.com, Subject=Price for 100mg x 60 pills $2.17 per pill]
The mail was tagged SPAM

134Nov 19 14:41:455510[AS][Pass][Category=NotSpam-L2, RefID=0001.0A090209.4923C332.0026:SCFSTAT1154798,ss=1,vtr=str,vl=0,fg][@S:R=13:1, 211.72.158.250:53038->61.7.147.149:25][SMTP][Sender=gareth_lin@draytek.com, Receiver=administrator@ruamkij.com, Subject==?gb2312?B?e7y8yvXIy2GyxbXEudx9wO28vGPE3A==?=]
 
This email is from our colleague, Gareth to Receiver=administrator@ruamkij.com  It is not a SPAM, so it won't have a Tag in the subject.

134Nov 19 14:43:085510[AS][Pass][Category=NotSpam-L2, RefID=0001.0A090203.4923C385.0015:SCFSTAT2070831,ss=1,fgs=0][@S:R=13:1, 203.81.21.13:4696->61.7.147.149:25][SMTP][Sender=cathy@asianmetal.com, Receiver=saiethong@ruamkij.com, Subject=from Asian Metal ]

This email from ][SMTP][Sender=cathy@asianmetal.com to  Receiver=saiethong@ruamkij.com, it's not a spam email
 
For more log you can check the file in the attachment.
 
So not all email need to check, before we set anti spam we need to clear where is email server, and which kind email we need to do antispam checking.