Exchange "Client does not have permissions to send as this sender"

You would need to configure "Send As" permission on your mail server to allow your agent accounts to send email as your outbound mailbox account. Below are instructions on how to do it in some of the more common email server/providers.

"Manage Send As Permission"

" in order for him to be able to send messages as another User "

Get-ReceiveConnector "Client" | Add-ADPermission –User "authenticated users" -ExtendedRights ms-Exch-SMTP-Accept-Authoritative-Domain-Sender

[PS] C:\>Get-ReceiveConnector -Identity "NS2\Client"

Identity       Bindings      Enabled
--------       --------      -------
NS2\Client  {0.0.0.0:587} True

Get-MailboxPermission -Identity "tim.lau" | fl

default permissions

To modify the default permissions for a Receive connector, you can also use the Add-ADPermission cmdlet

The Permissions Group that allows authenticated users to submit and relay is the "ExchangeUsers" group.

The permissions that aregranted with this permissions group are:

NT AUTHORITY\Authenticated Users {ms-Exch-SMTP-Submit}
NT AUTHORITY\Authenticated Users {ms-Exch-Accept-Headers-Routing}
NT AUTHORITY\Authenticated Users {ms-Exch-Bypass-Anti-Spam}
NT AUTHORITY\Authenticated Users {ms-Exch-SMTP-Accept-Any-Recipient}

The specific ACL that controls relay is the ms-Exch-SMTP-Accept-Any-Recipient.

Remove-MailboxPermission