How to set smtp relay from one exchange organization to another organization

Question

We have one company A and now it is splitting into two companies A and B.
We want to set up a relay so that when an email come from application to company B on-premise for the users who are still in Company A (mailbox not migrated to Company B) should go from B to A through relay connector using a particular DNS Namespace of company A.
We have added company B exchange servers IPs to DNS Namespace(SMTP server) of Company B. We are using exchange 2019.
how we can achieve this. Do we need to add Company A domain as accepted domain or we can need to connect Transport rule. how mail will flow from company B to company A using this particular DNS Namespace(SMTP server) of Company B.

Answer 1

@neelam Kumari

• Do you mean that mailbox hosted on company A (such as ******@domainA.com)?
• Whether there exists mailbox for this user like ******@domainB.com on company B?

Scenario one: If there doesn't exist ******@domainB.com on company B (which means **the original recipient is ****@domainA.com), just Domain A MX record points to Domain B IP Address. About this one, why not point MX to Domain A Ip address directly?

You can also use the Exchange external relay function to relay emails from company B to company A: Add domain A as external domain on company B, then create send connector to relay emails to domain A.

Scenario two: If there exist ******@domainB.com on company B (Which means **the original recipient is ****@domainB.com). You could take step below to redirect emails from ******@domainB.com to ******@domainA.com

• Add ******@domainA.com as mail contact on company B.
• Create a transport rule on company B like: If the message is sent to '@domainB.com', Redirect the message to '@domainA.com'

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Answer 2

Here scenario is like we want to relay the email from app server to company A exchange server and then company A exchange server should relay that mail to Company B exchange server (to company B user mailboxes on Company B exchange server)

Answer 3

is it Enough if we make below connectors to relay mail from App server to company B via Company A:

1. One receive connector on Company A and add APP server ip to the connector to receive mail from App to company A
2. One send connector on Company A and add Company B relay server IP address to send that mail (Came from app) to relay mail from Company A to company B
3. One receive connector on Company B and add Company A server IP address to receive mail from company A to Company B

Answer 4

yes, we created the connectors but while sending mail from telnet it shows "cannot relay email from non- accepted domain"