Using Custom Templates

[Tal]

I have a client who would like an email template to match their website
(much more basic) that they can use when they send out newsletters.

I can design the basic html template - what's the best way to get it to
work with outlook?

I figured out how to set a template - but it appears to use it for ALL
outgoing emails - no way that I could tell that you could just use it on
a one-off basis

Thanks