That's a pretty bad idea, unless you want to be an ESP. deliverability becomes a challenge when you start sending our massive quantities of email -- you need to deal with ISPs of all kinds, constantly changing legislation and complaints. one esp (my previous employer) does offer an API to tie your customer db to a mailing list (including doing the mail merge and such) -- verticalresponse.com
Current mailers are too complicated and generally don't have any APIs for instance to tie my customer db to a mailing list.