How do de-emphasis a sentence like this?

De-emphasize) Since we can send e-mail messages to only three offices—Knoxville, Charleston, and Baton Rouge--I will fax these documents to you after seven o’clock.

I put the dashes and commas in, but that is how you emphasis, right. My book says to emphais with dashes and de-emphasis with parenthesis. But how do you deemphsis with commas in the middle? Dashes and commas go together with de-emphasising but how do you deemphasis?

I have to answer this for homework and take a test. Help!