Why do certain lines within emails become underlined when received in yahoo?

I think those are what they call Yahoo Shortcuts.  Should just be actionable info, like phone#'s, locations, email addys, and if you hover over them I think it will give you a shortcut menu for various stuff yahoo might be able to do with such info.
Basically just yahoo gunk messin' up your work.

You can also test your emails using Litmus to see what the emails will look like before your send them. It is one of my favorite tools.