MTA, easy insertion of header line

[Brett G Lemoine]

I may have a case where I need to build an relay box for the
sole purpose of adding a header line into incoming mail
(specifically a content-type/character-set header) (don't ask).

Without starting an MTA religious war, what recommendations
would the kind folk on baylisa have on this subject?  I know
I could probably kludge it to work on sendmail (perhaps using
milter), but it's been too long since I looked at postfix, exim,
etc.. for me to recall if they can easily do something like this.

FWIW, the header content will be static.

thanks,
bl