Image proxy
A service an email provider uses to fetch and cache images on the reader’s behalf.
Gmail, Outlook.com and Apple Mail’s privacy features fetch email images through their own servers instead of the reader’s device. This hides the reader’s IP address and protects against tracking.
For signatures this means images must be publicly reachable at a stable HTTPS URL; images on private intranets or behind logins will not show. Proxies also cache images, so a changed logo at the same URL may take time to update.