Syncing three Apple Devices
How do I sync my iPad, iMac and iPhone so that when i delete an email on one device, it deletes on the other two?
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That depends more on what kind of mail setup you have for email
With IMAP it does what you want here.
POP however does not sync between devices.
You might want to check with your email provider if you are unsure what you got. And if you can switch to IMAP if you have POP
Mail settings you might need from your email provider - Apple Support