To relay emails from Exchange to ESP server, the general steps would be like:
1.add the recipient domain of the ESP server (for example contoso.com) as an accepted domain (external relay domain) in Exchange
For details please refer to this link: Accepted domains in Exchange Server
2.create a receive connector to receive emails from allowed ip addresses to be relayed
For details please refer to this link: Allow anonymous relay on Exchange servers
(It is suggested to only add the allowed ip addresses to the Remote IP addresses to avoid your Exchange server becoming an open relay)
3.create a send connector to send the emails to ESP server
For how to create a send connector, please follow these steps:
1.access Exchange Admin Center(https://<exchange FQDN>/ecp) and locate mail flow>send connectors
2.create a new send connector named ESP and select Custom as type
3.select Route mail through smart hosts
add the ESP server's ip address or server name as smart host
4.select Basic authentication and Offer basic authentication only after starting TLS, enter the credentials for authentication
5.configure emails sent to which recipient domain would be sent to this ESP server (in the below example, emails sent to recipients in contoso.com would be sent to ESP server)
6.select Exchange server as the source server
7.finish creating the send connector
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.