Why smtpd module is interesting ?

with an smtp module, apache could handle synchronous (http) and asynchronous (smtp) webservices.
with apr tools to parse and deal with xml, it could be easy to do some xml and webservices modules.
Using apr tools to contact db or ldap is also great to do something nice in the module.

How i see the module 

The module should first be able to handle communication with client, then receive the email data.
This data will go in different filters for headers parsing, then the module write data to the fs. 
As httpd do, filters could be used to deal with email content.   

for example, a web services module would just read data with an input filter, feed the apr_xml_parser,
then play with the parsed content. to send back data or xml.