Where do you see them as underexposed, back in LR after reimporting the Exported images, or in a browser, or where?
Your monitor profile is wrong. Lr and PS are fully color managed and use the profile to display; other applications may not be and will just ignore it.
Recalibrate, and don't change any monitor settings subsequently as that will invalidate the profile. If you don't have a calibrator use sRGB IEC61966-2.1 until you eventually decide to get one.
What "twenty_one" says in his post will get you consistent color display on your monitor across viewing applications, but not necessarily accurate.
If you need consistent and accurate display then you must calibrate and profile your monitor using a hardware device designed to perform this function.
When I presented the question, I should have included that I had already recalibrated the display monitor, and was well aware that this could have been the etiology for the problem. After much frustration I called Adobe for assistance. It took them about 1/2 hr. and finally found the problem to be associated with Windows. I am not exactly sure what they did, but my problem,(exporting images from LR5.7 to the hard drive or email resulted in under-exposed images, after being exported) has been resoved. Thanks for the advice.