Folder-level permissions will also allow you access, the difference with Full access permissions is that the former are more granular. If your case, the user has likely been granted permissions not only on the Calendar folder, so double-check that (either via Outlook/OWA or the Get-MailboxFolderPermission cmdlet).
User is able to open another user's Inbox folder without delegated Full Access

Hi All,
User A is delegated Send of Behalf and calendar Editor permission for a mailbox of User B.
User A is NOT delegated Full Access for a mailbox of User B.
It was found out that User A can open Inbox folder from desktop Outlook.
User A goes to File - Open & Export - Other User's Folder, then select user User B and folder Inbox and the entire Inbox folder of User B is opened in user's A Outlook.
How it is possible that User A can open User's B Inbox folder without having Full Access delegated permission?
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Outlook Windows Classic Outlook for Windows For business
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Vasil Michev 119.5K Reputation points MVP Volunteer Moderator
2023-12-21T08:11:42.46+00:00
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Anonymous
2023-12-21T08:21:34.9633333+00:00 Hello @Cloud_Geek_82
Could you provide a screenshot of user B's "Get-MailboxPermission" output?
In addition, you can also check the following settings to see if inbox is "None"(file->account settings->Delegate Access->user A->permissions)
In the "Delegate Access", please check whether user A has been granted "Inbox Permissions".
refer to: Manage another person's mail and calendar items
Regards
SF