I am representing a cPanel based web hosting provider having issues with images not loading through Apple Mail client on iOS/OSX in some cases; seems like something with a recent update as this is a new complaint and we haven't changed anything on our end.
This issue is reproduceable on multiple customer and technician devices, including but not limited to
- Phone SE third gen 2022 iOS 16.5 Beta
- Mac (Ventura 13.3.1 (22E261))
- iPhone iOS 16.4.1
All devices are running the latest available version of the Apple Mail app from the store.
To reproduce the issue all we have to do is send an HTML message containing an image with a src tag to a location on our server, and the image won't load in Apple Mail.
Other email clients work fine on the same device, and the user can visit the images directly using their web browser, and they load fine.
On the device side, we have confirmed that we are not blocking remote content, and that Apples mail privacy features are disabled. Cellular connection or WIFI, does not matter.
With testing, we've confirmed this is not related to the IP address, or SSL certificate directly and feel it has something to do with the HTTP headers or SSL cipher settings.
This was determined as a result of an A/B test with the same VPS server, same network, same IP address, same images, the only difference was that during the first test, Apache is used and the images won't load at all, and during the second test Nginx is used in front of Apache as a reverse proxy, which allows the images to load normally.
The exact same LetsEncrypt based SSL is used in both tests since it's the same underlying server with the only difference SSL wise being some different ciphers supported on Apache vs Nginx.
We're still trying to work with the cPanel team to compare the HTTP headers and see if we can't find a specific function or flag causing the problem, but if anyone else has any suggestions we'd be thrilled to hear them as this issue is causing our clients grief.