Device token is obtained on startup via Android APIs if you use Google Play version, in F-Droid it is disabled. Then when you connect to chatmail server this is discovered via IMAP capability string XDELTAPUSH and stored using IMAP METADATA extension on the server. When a new message arrives, if the token is stored, it is relayed to notifications.delta.chat service and it notifies the device that Delta Chat got new messages. Delta Chat then checks all accounts on the device (because notifications.delta.chat and Delta Chat application do not know anything about email accounts).
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