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When one device tells
an IMAP server to delete
a message, the change
propagates to all others
Why are messages deleted using my iPhone also removed from my Mac?
QI have an iPhone 5 running iOS 9.2.1 and bought a new iMac running OS X 10.11.3. When I perform any actions in Mail on messages in my BT Internet mailbox, such as deleting one, that is replicated on the other system too. I often read and delete emails on my phone, only to find that they have also been deleted on my Mac. How can I fix this?
AThe behaviour you describe is correct for IMAP, the protocol you are using to connect to your BT Internet account. IMAP servers store messages on the server, enabling you to access the same mailboxes from any device you’ve configured to access your account, such as your iPhone and your Mac. When one mail client, such as Mail on your iPhone, tells the server to delete a message, all devices which access the same account see the change and the message is deleted on all of them.
For anything different to happen would require each device to see the mailbox differently, which would rapidly become too complex to understand: you might have to delete a message several times before it would finally disappear from all of your devices.
If you want to retain a message for access later using a different device, you have two options: you can leave it where it is, and deal with it on that other device later on, deleting it when you’re finished with it; or you can move the message to another mailbox, from which it’ll be accessible on the other device. You can also create local mailboxes to store messages, which removes them from your IMAP mail server and so hides them from your iPhone.
Moving messages to a local mailbox on your Mac lets you keep copies while getting rid of them from other devices.