How to extract List of all Users Who Have Full Access, Send As and Send on Behalf to Other Exchange Mailboxes.

Hi @Mohammad Haque ,
According to my research, we cannot directly obtain the permissions of the user's mailbox to other mailboxes.We can know whether each user mailbox contains the "Full Access", "Send As" and "Send on Behalf" permissions of other mailboxes through the permissions assigned to other users on each user's mailbox.

We could run the following script to get the permission between user mailbox:

Full Access:

$mailboxes = Get-Mailbox -ResultSize Unlimited -Filter ('RecipientTypeDetails -eq "UserMailbox"')  
  
foreach ($mailbox in $mailboxes)   
{ Get-MailboxPermission -Identity $mailbox.alias -ResultSize Unlimited | ?{($_.IsInherited -eq $false) -and ($_.User -ne "NT AUTHORITY\SELF") -and ($_.AccessRights -like "FullAccess")} | select Identity, User, AccessRights }  

Send As:

$mailboxes = Get-Mailbox -ResultSize Unlimited -Filter ('RecipientTypeDetails -eq "UserMailbox"')  
  
foreach ($mailbox in $mailboxes)   
{ Get-ADPermission -Identity $mailbox.alias | ? { $_.ExtendedRights -like "*send*" -and -not ($_.User -match "NT AUTHORITY") -and ($_.IsInherited -eq $false)} | select Identity,User,ExtendedRights }  

Send On Behalf:

Get-Mailbox -ResultSize Unlimited -Filter ('RecipientTypeDetails -eq "UserMailbox"') | select Alias, GrantSendOnBehalfTo  

The following screenshots in the test in lab environment:
Full Access:

Send As:

Send On Behalf:

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The one for SendAs in Exchange online is the following. Code below for shared mailboxes

$mailboxes = Get-Mailbox -ResultSize Unlimited -Filter ('RecipientTypeDetails -eq "SharedMailbox"')  
foreach ($mailbox in $mailboxes)   
{ Get-RecipientPermission -Identity $mailbox.alias | Where-Object { $_.AccessRights -like "*send*" -and -not ($_.Trustee -match "NT AUTHORITY") -and ($_.IsInherited -eq $false)} | select Identity,Trustee,AccessRights }

How can we then export to csv.

Melvi