In photoshop all you need to do is use save for web and devices. On the bottom right hand side of the dialog box is your pixel dimentions. Just lower it to 1600x1200 or less for emails. This dialog box will strip the ppi from the metadata and other metadata if you want it to.
or
In the upper right hand corner of the save for web dialog box is a small icon, clicking it will bring up a menu from there you can choose Optimise for file size. In that dialog box you can type in a file size you prefer. (This is optional)
If you would have specified your operating system, someone may be able to tell you how to auto reduce images when sent to the email app. I know Windows has this feature, I do not know about Mac's.
And there are several other ways including converting a folder of images at once. But the above should suffice for now.