Loading internal server images when mail is sent from external domain

Loading internal server images when mail is sent from external domain

Our company has an intranet intranet.example.com.
When I send emails from outlook that contain images from
intranet.example.com they load in Outlook, but not on my iPhone. On the
iPhone I get a blue box with a question mark.
When I send the exact same email from my internal mail then the images
load without problem on both the iPhone and also outlook.
Lastly, if I send an email to my iPhone where the internal image is
replaced with an external image, say Googles logo loading from google.com
domain then my phone displays this correctly.
The issue is that we have a 3rd party company that needs to send us an
email and our server ops guys say it is going to take 4 - 6 weeks to push
out their domain to the safe senders list. The problem is, if they host
the images externally then the internal outlook client doesn't load the
images.
I don't understand what is blocking the internal images from loading. I
assume there is some cross domain thing going on with the sender and the
image location, but it sounds weird that I can send from outlook to show a
Google logo without problem. Is there anything like this that can be
pushed to company phones?
We are using the standard iPhone mail application.