For any sort of formal or business writing, I recommend you avoid the exclamation mark.
If you want to emphasize your point, consider using bold or italic instead. The text will stand out more.
The problem with them is that unlike emoticons, they can indicate humour, or anger, or irony – and it’s not always easy for the reader to tell which of these you’re trying to convey. The potential for misunderstanding is huge. For that reason, I recommend you avoid them and use words instead.
If you feel compelled to point out that something you’ve written is witty or ironic, then use a single exclamation mark. But remember it’s a bit like laughing at your own jokes! How funny is that?!!
This isn’t to say you should never use them. They can be useful for giving orders, or urgent instructions: Stop! Children for example. Or Don’t forget to buy milk! Just try and restrict them to informal email and texts, and even then use them sparingly for maximum impact.