Software for email merging

["massimo.modica@tin.it" <massimo.modica@tin.it>]

Hi,
I'm doing a free software migration and I need to replace an application 
for email merging called World Merge:

http://www.coloradosoft.com/worldmrg

It's used to send customized newsletters to firm's clients.
Does anybody know about an any existing - or in development - equivalent?

Thanks.

Massimo