Okay. I did some more testing following the latest GP theme update v188.8.131.52
What I found was the 600×9999 images still generate if I have both MailPoet and GP Premium activated.
I also noticed that when GP Premium is activated I can not generate proportional images.
I had this issue before starting this thread however I had deactivate sizing (by setting to 0) in settings / media so hadn’t realized GP Premium was the cause this issue until today.
(NB “Crop thumbnail to exact dimensions (normally thumbnails are proportional)” is unchecked during these tests.)
If I reactive media sizing with GP Premium activated, I get hard cropped and even stretched images if the sizing number is larger than the original image. I also know this was happening prior to the latest GP theme update v184.108.40.206
While both MailPoet and GP Premium must be activated to make the 600×9999 images, I only need GP Premium activated to stop wordpress from generating proportional images.
MailPoet plays no part in stopping wordpress proportional image generation.
There certainly appears to be some sort of conflict with GP Premium and WordPress in this respect and I would assume at the stage that MailPoet is not really the issue here.
Where to from here? Any solution suggestions you could implement in GP Premium would be awesome.