Can anyone help identify this movie?

My daughter is telling me we watched a Western a few months ago, but I'm drawing a blank on film it is!

In the film a cowboy and his older friend go to trade with native Americans and as part of the deal through some misunderstanding the cowboys end up with a woman from the tribe, who thinks she's now his wife.. there's a lot of letter writing in the movie apparently.

It's an older film she thinks.

It sounds familiar but I can't place it!