Your company email signature incl. logo and other images.
When you reply or forward an email, sometimes email signature images are missing and replaces with image tags at the recipient end:
<image001.jpg> <image002.jpg> etc.
For the signature images to work correctly on Mail app for iOS the images must be linked.
This is default all major email clients like Gmail, Outlook on the web, Mail on OSX and Outlook on OSX, but not default in Outlook on Windows.
When emails send from Outlook on Windows and replied from iPhone, the images are removed because images are embedded in the email.
This is a great feature as no Red X will show before the images are downloaded - Except it doesn't work when replied/forwarded using Mail for iOS:
Use the app 'Outlook for iOS' to fix this or you can switch to Android, Outlook on Mac or use Outlook on the web (incl. with Office 365).