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So it is easy to find out what USER is a member of what GROUP. Or vice-versa.

What is not as easily available is finding out what USER has access to another account. Or another way of putting it is; how to find mailboxes that have additional permissions than just their own?

Here's how:

Get-Mailbox | Get-MailboxPermission | where {$_.user.tostring() -ne "NT AUTHORITY\SELF" -and $_.IsInherited -eq $false} | Select Identity,User,@{Name='Access Rights';Expression={[string]::join(', ', $_.AccessRights)}}

Or if you need to create an Excel document out of it:

Get-Mailbox | Get-MailboxPermission | where {$_.user.tostring() -ne "NT AUTHORITY\SELF" -and $_.IsInherited -eq $false} | Select Identity,User,@{Name='Access Rights';Expression={[string]::join(', ', $_.AccessRights)}} | Export-Csv -NoTypeInformation mailboxpermissions-v1.csv

Notes:

This is found in the article for Remove Mailbox Permissions That Are Not Inherited but the info might be hidden there.