Scenario
You are using the classic version of Microsoft Outlook, and notice that images or weblinks are displayed in the following manner:
{HYPERLINK "mailto:user@domain.com"}
Solution
Pressing the Alt + F9 key combination in classic Outlook will enable the Document Properties feature. This is an inbuilt feature of Outlook and Word and is not controlled by Signature 365.
Pressing this key combination again will disable this feature.