Gracious and aloof.
Those are the watchwords I usually attempt to embrace when I'm annoyed, irritated, hurt by a friend's/associate's words or deeds.
I'd certainly confide in "mutual" friends about my concerns and feelings about something, but I think vixmom and Druxy and Kerry are correct...the strongly worded e-mail will not solve -- but will accelerate deterioration of -- whatever the situation is.
If this is strictly business (i.e., no bonds of friendship ever involved), then strongly worded is the way to go.
My 7 1/2 cents worth.