ActionMailer v2.x changes to formatting?

I've been trying to track down a formatting problem in my plain text
emails that I'm sending out. After a couple hours, I think it might be
due to the version of ActionMailer that's being used...

The problem is that when sent from my dev machine all's good, but when

I'm actually still developing using rails 1.2.6 on my dev machine, and
setting RAILS_GEM_VERSION to this version but the deployment
configuration has rails 2.x and other gems installed alongside the
older gems. So I ~think~ that's coming into play.

Any thoughts about whether I'm on the right path in thinking that the
new ActionMailer is being utilized and whether, in fact, it's
formatting output differently?

Also - If anyone knows if it's possible to set the version of other
gems ala RAILS_GEM_VERSION can you point me to where I can learn how?