I think I can manage the size and bandwidth of the images I'm sending via emails myself, but any time a large enough image is attached to a new email, e.g. via Windows Explorer right-clicking an image → Send as Email, Thunderbird asks me if I want to resize the image.
It has caused more trouble than benefit in the past, e.g. instructing some users to send some important scans via email, which would then send them in the downscaled size instead of the original one.
How do you disable this option in Thunderbird? My version is 11, but this option exists for previous versions, too.