I have two completely distinct Zimbra OSE installs for distinct domains.

Is there a mechanism for user A on domain/install 1 to grant read/write share access (for a calendar, address book or email folder) to user B on domain/install 2?

Trying to do it as 'share calendar to Zimbra user' from userA@domain1 states that userB@domain2 is not a Zimbra user (ok, I can understand this as there isn't any connection established at the server level).

Trying to do it as 'share calendar to External Guests' generates the email from userA@domain1 to userB@domain2, but when userB@domain2 tries to 'accept', domain2's Zimbra complains about 'No such account exists', and details show what looks like a UUID - which makes me suspect that domain2 is TRYING to accept an internal Zimbra share but failing because (just as in the case of 'share to Zimbra user') the two servers aren't connected in any way, but it expects them to be.

Looking at the share notification email, it looks like the piece that Zimbra is actually looking at is the MIME part: xml/x-zimbra-share. In this I see two problems:

1.) the <grantor> tag uses an ID that looks like a UUID rather than a mailbox name (so how would the receiving email server have any clue what this refers to)
2.) the <link> tag uses a resource ID (I think this is the same ID that you would use in zmmailbox to look at the resource) that is specific to domain1's Zimbra install - the same ID on domain2 clearly won't refer to the same resource.

So what would the answer be? Perhaps if Zimbra could act as a full CalDAV/CardDAV client as well as server? That wouldn't address email, but emails could be addressed as IMAP I suppose.