Spam checking software looks for occurrences of words that are often used in spam. For example, if it was looking for the word "Viagra" it would not find it in this email. The exclamation point looks like the letter "i" to the human eye, but not to a computer. They may substitute the "@" for the letter "a" the next time. There are too many possibilities of word combinations as it is to check all the other possible variations.
Hi,
6eneric Cia|is & 6eneric V!agra
b0th avai|able 0nline!
y0u know about V!agra, but if y0u haven't heard
of Cia|is, the weekend pi||, y0u need to try it
It's als0 known as "Super V!agra" b'cause it w0rks
right away and lasts up t0 36 h0urs!
We have b0th 6eneric Cia|is and V!agra. Same ingredients, much cheaper pr!ce!
http://.....
Letters arranged vertically cannot be picked up by computers that examine words horizontally.
BUY!
V O
I N
A L
G I
R N
A E
Click here http://....
There are also too many ways that letters can be spaced apart, with blanks or other characters
B U Y V I A G R A !
B*U*Y V*I*A*G*R*A