Is it OK to HTTP redirect images used in email clients?

Question

I have seen the answers to Is it OK to HTTP redirect images. It is a YES - browsers would follow redirection.

However, do email clients (GMail, Express, Thunderbird, Yahoo...) follow the same rule?

Answer 1

I just answered this at superuser. Here is the same answer:

Yes. I just sent an email containing <img src="http://www.example.com/img/logo.png" alt="not displayed"/>. The example server is in my control in redirects to the same URL with https.

My findings:

• GMail on 24 Oct 2017: It displays the image, but replaces it's location to something like https://ci3.googleusercontent.com/proxy/2ks...5hk=s0-d-e1-ft#http://www.example.com/img/logo.png. I guess that all browsers follow redirects and therefore all webmail interfaces should be fine.
• Thunderbird 52.4.0: It doesn't load remote content by default, but after allowing it, the image gets displayed correctly.
• Evolution 3.18.5.2: Same as Thunderbird. Once you load images via Ctrl+I, it's displayed.
• Outlook (unknown version): A colleague confirmed that it's displaying on their Windows machine.
• Mail on macOS (10.3): Another colleague confirmed that it's displaying when they re-open the email. It might just be that the image took a few seconds to load, not sure.