View delegated mailbox permissions within Exchange Server or Exchage Online

To generate a list all mailboxes within an Exchange organisation that has other users assigned Full Rights to them, you can perform the below Exchange Management Shell or Exchange Online command:

Get-Mailbox -ResultSize Unlimited |
    ForEach-Object -Process {
        $UserPrincipalName = $_.UserPrincipalName

        Get-MailboxPermission -Identity $UserPrincipalName |
            Where-Object -Property User -NE -Value "NT AUTHORITY\SELF" |
            Select-Object -Property @{
                Name       = "Mailbox User Name"
                Expression = { $_.Identity }
            },
            @{
                Name       = "Mailbox User UPN"
                Expression = { $UserPrincipalName }
            },
            @{
                Name       = "Delegate User UPN"
                Expression = { $_.User }
            },
            @{
                Name       = "Delegate User Access Rights"
                Expression = { $_.AccessRights }
            }
    } |
    Sort-Object -Property "Mailbox User Name" |
    Export-Csv .\MailboxFullAccessPermissions.csv -NoTypeInformation

This will export a MailboxFullAccessPermissions.csv file current directory that will list the Mailbox Name, UPN and Delegate User UPN and the Access Type.