If you have some sort of 3rd-party application that needs access to all of your mailboxes, it can be a pain to set the permissions on all of your mailboxes.
Except if you use this nice little powershell command:
Get-Mailbox -ResultSize unlimited | Add-MailboxPermission -User admin -AccessRights FullAccess -InheritanceType all
What this command will do is to add Full mailbox access to every mailbox in your Exchange organization for the user admin.
You can of course change the -AccessRights setting to your liking, e.g. SendAs.
You can also narrow down the number of mailboxes affected by changing the search parameter after Get-Mailbox
For example if you only want to set SendAs permission on a mailboxes in a specific OU for the user admin:
get-mailbox -OrganizationalUnit domain.local/ou/Users/ | Add-MailboxPermission -User admin -AccessRights SendAs -InheritanceType all