Q: why does the camera on iPhone take multiple pictures when I press the button once?

1. When you press down the button for too long you take, what's called, Burst Photos. It's done so you can choose the best one from say, a fast action sequence. Practice a quicker tap on the button to avoid this.

2. Please explain in more detail.

That's something you would have to ask your email provider.

Note also that you can use the left-side volume buttons to take the photo. While in the camera they work like the shutter button, with the benefit of giving the tactile click feedback.