Sending e-mail based on templates?

I would like developing an application for sending e-mail based on a (html)
template. Scenario: user chooses distribution list and chooses a template
(fro ma list of available html-templates). These templates normally will have
tags that should be replaced with data (eg [FirstName]). This data is
provided by the distribution list. I know when the distribution lists and
templates are static (= the template will always use the same tags) you use
string.Replace and solved. But ...
what I want to achieve is that the user can creta his/her own templates with
fields from the db tables. this means the templates are not "static" nor are
the queries to create the distribution lists. My question now: what is the
best way to achieve this kind of "dynamic" templates. Should I put the used
tags in the template in a db table as well as the respective
database-table(s)-column(s) or in an xml file? Or what is the best way to
solve thi kind of problem?

Thanks.