With today's 10.1.7 PTR build, Blizzard has added a new setting that allows players to restrict calendar invites. Calendar invites have been used lately by scammers to reach players as other contact avenues have been closed over the years.
By default, this setting is on, meaning if you want to receive invites from strangers, you need to turn it off once Patch 10.1.7 hits. With it enabled, you can only receive calendar invites from Friends or Guild Members.
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