The connectedaccounts feature in Office365 is a mechanism by which users can configure connections to multiple external accounts over IMAP or POP3 protocols and bring those messages into their Outlook mailbox in Office 365.
This feature also enables organizations to implement Exchange Online during the Pilot Phase of a FastTrack deployment, providing users with the experience of receiving and managing email in their mailboxes, yet reverting to the existing email
system if the organization does not decide to go ahead with the deployment. However, if you carry out a mail migration to Exchange Online, the connected account is merged into the new Exchangeonlinemailbox.