We're using a legacy Order Management System called Stone Edge Order Manager which allows us to send emails to customers with data populated from their account and order information.
It has a very basic templating system that allows us to edit the emails before they are sent to add additional information. Unfortunately the HTML support is very inflexible and does not allow us to edit messages when sent in HTML format. That means we can only send plain-text emails to customers.
We would like to be able to send professional looking emails to our customers and already have a HTML template we are using in other software.
There are two possible ways I can think of to achieve this goal.
A SMTP server that allows us to modify messages being sent via the SMTP server. Something that can prepend a header and append a footer into the message and change the mimetype.
An external email client software that can apply a HTML template to emails sent. The Order Manager software supports sending via the default email client setup on the workstation.
I've tried searching for more details on both possible solutions but I'm having trouble finding any relevant results. Surely I'm not the first administrator to want to do this?