I believe the 5 MB is the total of all attachments and each must be much smaller. The software I have for my old camera doesn't give me a lot of options so I go to this site to reduce my file sizes:
https://www.reduceimages.com/
I just use their defaults and download it from there and have never had a problem uploading. It's just a matter of dragging your image to their site and click a couple of buttons.
I do this now for all my images, so before I send an email to someone it is already reduced. I can tell you that my emails with images get sent about 3 times as fast now! For people using smart phones there are defaults and options you can set. Again, I'm using an older digital camera so I have to take this extra step.