Under certain conditions the SEND TEST EMAIL functionality will not work due to restrictions on the exchange server.
This method of enabling the SEND TEST EMAIL functionality involves allowing the IIS server to relay through the Exchange server.
If you need to enable the SEND TEST EMAIL functionality on multiple exchange servers, this procedure must be repeated on those servers as well.
Create a new inbound connector
In the ECP, goto the Mail Flow settings and click on the + to create a new connector for the selected Exchange server.
Select the role and Type
Select the bindings
You may need to add the IP6 addresses to you connector. (Mail Signature will only operate on port 25)
Set the IP address for the IIS Server hosting the Mail Signature Application
Save and close the connector
Save and close the connector, the reopen the connector.
Set the Authentication
Mail Signature will attempt to authenticate if the SMTP allows it, this can cause the IIS to attempt to authenticate as the security context the Application Pool runs under.
Allow the connector to be used as a relay
Open the Exchange Management Shell and enter the following command:
Get-ReceiveConnector "MailSignatureNZEX01" | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "Ms-Exch-SMTP-Accept-Any-Recipient"