jfaughnan wrote:
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4. I believe one can now have an Apple ID that does not use a .me or .icloud or .mac domain.
There are two reasons that you should never trust a calendar invitation to work between any provider to any other provider. The ONLY time you should expect it to work is within a specific technology domain such as users that are ALL within a single organization that uses a single Calendar server.
1) Per stated in (4) above, the email address that is very loosely related to any given AppleID has nothing to actually do with the AppleID account. The username of every AppleID is (unfortunately) an email address that the user has provided when creating the account, yet is has NOTHING to do with the AppleID nor the email/calendar provider. Therefore, there is absolutely no way for Apple to know if the email address that is provided when creating an AppleID actually has any capabilities to receive calendar invites. In the most obvious example, just because I use a .iCloud account when creating an AppleID, doesn't mean that I either use iCal or that I even have iCloud Calendar enabled at any on any of my Mac OS or iOS devices. It's a very troubling issue that nearly all "cloud"/Social service providers use a provided email address text value as the userID of their accounts. I boggles my mind why cloud/social services every thought that having a persons unrelated email address as a username for an unrelated service was a good idea. For example, why in the **** would Google, Apple, Microsoft, Facebook want to allow a user to have a username of "xxxx.aol.com"? It makes no sense to even the most knowledgeable users and completely confuses all others! I'll say it one more time, using email addresses as username makes no sense at all but it seems that Apple along with all the big players are still doing it so we are stuck with it.
2) Even in 2016 standards for email, calendars, and contacts are still unstable and can;t be trusted for interoperability. Anyone that insists on still using client applications for email, calendars, or contacts in 2016 understands that painful reality that IMAP, CalDAV, and CardDAV or vCard are still far from standards that actually WORK across clients. The reality is that they don't and you should expect that exchanging such information between clients will never work reliably as long as providers insist on using proprietary interpretations of "standards".
Therefore, there is no solution to expecting that iCloud calendar invitations will ever work as expected unless ALL the users involved are within a managed domain that has a controlled Calendar client environment (i.e. all of your users are using Apple devices and Apple servers). Otherwise, the only trusted solution is to send an email with the meeting details instead and allow the recipient to add it to their native calendar server via their native calendar client.
