When you send mail:

It leaves your computer and goes to the mail server.  It checks:

• Is the mail actually from you - it checks for spoofed addresses
• Is there a virus in it
• Does it contain any phrases or words that we block
• Does it contain an attachment - and if so it it safe and allowed
• Does the person you are sending to exist

The mail goes out.

When you recieve mail:

• Is it addressed to a legitimate address ( do you exist on our system)
• Is the sender in a safe sender list - if so below is ignored except for  real time blocking lists
• Is the sender listed in any real time blocking lists - Spamhaus Spamcop
• Is the sender legitimate - It asks the sending server if the from is valid (SPF check)
• Are there attachments and are they allowed
• Are there blocked words or phrases
• Is there a virus
• Does it match the pattern of spam - it creates a score and if >60 it is flagged as spam
• Is the sender in our or your blocked sender list

Its delivered