Use Words for Emphasis, Not Exclamation Marks
15% off Sale This Week Only!!!!! Act Now to Take Advantage of This Great Opportunity!!!!!!
When was the last time you opened an email with a subject line similar to that one? It's likely your answer was something like "awhile ago," as most of us have wised up about the sneaky tactic spammers use, and such an abuse of exclamation marks is highly suspicious. But if you're not a spammer, you should definitely avoid looking like one, which is why you should use exclamation marks, especially in combination with the common spam buzzword "free," very sparingly.
When you're writing copy for your email marketing campaign and find yourself tempted to use an exclamation mark, stop, reread, and rewrite the content in a way that will make the sentence's structure and rhythm convey the sense of urgency you're aiming for.
If you're still not convinced by the fact that Content is King and words can be much more powerful than punctuation placement, consider English writer Terry Pratchett's take on exclamation marks: "Five exclamation marks, the sure sign of an insane mind."