Etiquette question - work related

[IndyColtsFan]

Question for the people out there who have had this happen. Let's say your boss sends an e-mail to you, and makes a grammar mistake that's on your pet peeve list, for example "for all intensive purposes" instead of "for all intents and purposes".

Here is where the etiquette comes in: do you correct them, or do you let it slide, allowing them to continue to make this mistake to other co-workers?

Let it slide. I had a boss that repeatedly said "It is a mute point" rather than "It is a moot point." I just laughed it off.