I have recently been exporting pictures from LR6, using, as far as I know, the same settings I have used in the past. I do no use a “preset.” I export to a folder, rename the files,, format as a jpg with sRGB color space and quality of about 80. I “resize to fit long edge” at 800 pixels and a resolution of 72 psi; I sharpen for screen at a “standard amount”.
The problem is that in a somewhat random fashion, many of these images — when I attach them to an email — are distorted. They seem to be elongated or “stretched”.
Are any of my settings inappropriate or is there some other problem that you are aware of?
LR has a problem with portrait images using the Long side option. For some reason it always reads the bottom edge as the Long Side. That isn't the case with images shot in the portrait orientation. So I suggest you stop using that option.
Use the Width & Height option and specify the 800 pixels on the long edge, W or H. That means you will have to separate your landscape shots from the portrait shot into 2 different exports. Or separate them, L - P and use the Long Side for the Landscape and the short side for the Portraits. But I'm not sure that will work.