We use a standard html email template for all our users, which is formatted via a table.
One user can "see" the table borders when he composes an email (i.e. in the same way as you can see table borders in word, even when no line style is applied.) For all other users they are invisible.
I'm sure this must be some preview setting, but I can't find it!
EDIT: Added picture as it explains it a bit better!