Although my training tells me not to overuse exclamation points because they are shouty and juvenile, I find myself using them because I fear being seen as unfriendly or insincere if I only use a period.
( Source: NYMag.com | It's Too Late. Exclamation Marks Are Unstoppable )
When I see emails, especially from folks I don't know, with no emotion (and that's a tough ask coming in the form of the written word), it's boring. I tend to think their topic is boring. And if what they are writing is coming across as boring, they must be bored.
For me, the exclamation point adds a bit of personality to an otherwise drab communication medium. Since I don't use stationary or type or fonts, it adds a bit of depth to an email message. A message with a bit of spice is much more appreciated, and likelier to be responded to.
Who wants to be boring?
Of course, not every sentence requires an exclamation point. Or every email message, for that matter! But strategically placed, they can liven up and convey your enthusiasm about a particular topic.