What is symbol?
Wondering what this is on home page
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That’s an Apple feature known as Continuity.
You were using Mail on another device.
Continuity allows you to switch devices and pick up where you left off.
Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
Specifically in this case, Handoff:
Use Handoff to continue tasks on your other devices - Apple Support
