DeltaSync was a proprietary Microsoft communications protocol for synchronizing web services with offline clients. It was switched off by Microsoft on 30th June 2016.
Windows Live Hotmail was connected to offline clients using DeltaSync; Outlook Connector and the Windows Live Mail client use it for offline access. Microsoft pushed for this protocol instead of POP3 or IMAP4 for supporting various advanced features, like contacts, calendar, and notes synchronization. Using the DeltaSync protocol, Microsoft intended to provide a common platform for any compatible device to implement a client which can two-way synchronize mail, contacts, calendar, and notes.