Increasing attachment size in mail

I have been building SMTP mail server using asp and c#. How can i increase attachment size in my smtp mail server.

Currently i can attach files up to size of 5mb, but not more than  that. I want to increase this size

I have tried adding tags in web.config file, . "httpRuntimeexecutionTimeout" and "maxRequestLength"  but that had changed nothing

please help me

 

here is  my web.config file
<?xml version="1.0"?>
<configuration>
<configSections>
...