<!-- PR description-->
As part of handling share mailbox-
Introduced new flag - `mailbox` and deprecated existing `user` flag
This is to handle wider group of mailbox types.
#### Does this PR need a docs update or release note?
- [x] ✅ Yes, it's included
#### Type of change
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- [x] 🌻 Feature
#### Issue(s)
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* #<issue>
#### Test Plan
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- [x] 💪 Manual
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