I used this powershell to quickly export Mailbox permissions from one mailbox and set the same permissions on another mailbox.
$source_mailbox = "[email protected]"
$target_mailbox = "[email protected]"
#Exporting full access permissions
Get-MailboxPermission -Identity $source_mailbox| Where-Object {($_.IsInherited -eq $false) -and -not ($_.User -like "NT AUTHORITY\SELF")} | select Identity, user | Export-csv -path c:\temp\mailboxusers.csv
$Mailboxes = import-csv c:\temp\mailboxusers.csv
foreach ($mailbox in $mailboxes) {
Add-MailboxPermission -Identity $target_mailbox -User $mailbox.user -AccessRights FullAccess -InheritanceType All -AutoMapping:$true
Add-RecipientPermission -Identity "$target_mailbox" -AccessRights SendAs -Confirm:$false -Trustee $mailbox.user
}
#----------
# if you want to export the share permissions separately
# ---------
Get-MailboxPermission -Identity $source_mailbox| Where-Object {($_.IsInherited -eq $false) -and -not ($_.User -like "NT AUTHORITY\SELF")} | select Identity, user | Export-csv -path c:\temp\mailbox-sendas-users.csv