I was reading this thread on Apple’s discussion forums and realize that figuring out what can synchronize to what when it comes to mail, calendar, and contacts can be pretty headspinning.

Here’s the thing: there are multiple protocols for synchronizing mail, calendar and contacts, variously supported by different servers and client software. I’ll try to explain.

The Summary:

Mail will always synchronize with whatever software or device you’re using.

However, for calendar and contact sync:

Google, iCloud, Yahoo, AOL, OS X Server calendar and contacts:

• Will sync with mobile devices
• Will sync with Mac computers
• Will sync with some Windows computers, depending on version and software being used

Exchange Server, including Office 365, calendar and contacts:

• Will sync with mobile devices
• Will sync with Mac computers
• Will sync with Windows computers

Outlook.com (Hotmail) calendar and contacts:

• Will sync with mobile devices
• Will NOT sync with Mac computers
• Will sync with Windows computers

The Ugly Detail:

Open protocols:

• IMAP: Syncs mail. Supported by countless software and provided by almost every kind of mail server, including Exchange, OS X Server, iCloud, Google, Yahoo, AOL, etc.
• CalDAV: Syncs calendar. Developed by Apple. Supported by modern mobile platforms, and some desktop software, including Mac iCal/Calendar since Leopard. Provided by iCloud, OS X Server, Google, Yahoo, AOL, and some other servers (but not Exchange or Outlook.com).
• CardDAV: Syncs contacts. Developed by Apple. Supported by more recent versions of modern mobile platforms, and some desktop software, including Mac Address Book/Contacts since Snow Leopard. Provided by iCloud, OS X Server, Google, Yahoo, AOL, and some other servers (but not Exchange or Outlook.com).
• POP: Downloads mail to inbox, but that’s all. No synchronization, no folder awareness. Not recommended for modern living unless you exclusively use one well backed up computer, and no devices. Provided by almost every kind of mail server, including Exchange, OS X Server, Google, Yahoo, AOL, etc., and occasionally POP is the only option available (e.g. the freebie mail you get with a GoDaddy domain, some ISP’s, and some older web hosts).

Microsoft protocols:

• Exchange ActiveSync (EAS): Syncs mail/calendar/contacts/tasks/notes. Almost exclusively supported only by mobile device clients. The only desktop clients on any platform that supports EAS are Outlook 2013 for Windows and Windows 8 Mail/People/Calendar (and Outlook 2013 will only allow connecting to non-Exchange EAS services such as Outlook.com and paid Google Apps). Provided by Exchange 2003 and later, Office 365, Outlook.com (Hotmail), paid Google Apps, and some other servers.
• (Since EAS was designed to be lightweight, it is more limited in its capabilities and how much data it communicates than other protocols; this may be why it is not widely used by desktop software, and  I suspect but have no evidence that Microsoft may have restricted EAS use in desktop software through licensing restrictions.)
• Exchange Web Services (EWS): Syncs mail/calendar/contacts/tasks/notes. Not supported by any mobile device clients. Desktop clients I’m aware of that support EWS include Outlook for Mac 2014, Outlook for Mac 2011, Entourage 2008 Web Services Edition (aka version 13), and Apple Mail/Calendar/Contacts/Notes (since Snow Leopard; Notes since Mavericks, I think). Outlook for Windows does not use EWS. Provided by Exchange Server 2007 and later, and Office 365.
• MAPI: Syncs mail/calendar/contacts/tasks/notes, plus all other information available from an Exchange server. It is the “native” connection protocol for Exchange since 1997 despite Microsoft’s attempts to get away from it. Only supported by Outlook for Windows (all versions), though Outlook 2001 for Mac OS 9 also supported it. Provided by all versions of Exchange Server, Office 365, Outlook.com (Hotmail), and paid Google Apps.
• WebDAV: Syncs mail/calendar/contacts, slowly and badly. Now deprecated. Not supported by any mobile device clients. The only desktop clients on any platform that support it are Entourage 2004 (aka version 11) and 2008 (aka version 12). Provided by Exchange Server 2003 and 2007.

API access to contacts:

• API for Gmail, FaceBook LinkedIn, Yahoo: Syncs contacts. These services have an internal proprietary method for accessing their contact information. OS X Leopard through Mountain Lion can utilize it to synchronize contacts with Google and Yahoo. On Mountain Lion through Yosemite, you can use CardDAV instead, as well as synchronize with FaceBook and LinkedIn using their own internal methods.

(Image by Jonny Hughes, courtesy Flickr Creative Commons.)