I've been keeping F12 (Developer Tools) open as I open emails. I have noticed several emails that all are <img src="/............" /> where there is no domain, etc. This is causing the developer tab to show tons of 404 messages.

This isn't tied to just those. There are also 403 messages appearing as well. 

I am not sure on being able to fix the 403 but the 404 being they are referenced the way they are might be able to be fixed within code.