IOS images in email signature intermittently replaced by NULL

ios
#1

I’ve been trying to find a reason to this problem.

I have created an HTML email signature in IOS containing images. When using the IOS mail app, all appears fine for a few days, but then when I use the email app again, the images have disappeared and are replaced with boxes where the images should be!

The images are linked using IMG SRC=

Anyone know why this happens?