If recipients are receiving your emails with missing images, broken signatures, or a strange attachment called winmail.dat, the cause is almost certainly that Outlook is sending your messages in Rich Text Format (RTF) rather than HTML.
Apple Mail (on both macOS and iOS) is designed to follow a principle called "send what you compose." The email client determines the format of an outgoing message based on the content it detects in the compose window, not on a blanket setting.