Migrate Zimbra to Exchange Online

Hi all,
Im going to migrate Zimbra to Office 365. So have to configure mail flow coexisting between Office and Zimbra for user in 2 email system can send message to each other.

Both of Zimbra and Office 365 using the same smtp domain like abc@mydomain.com. And the MX record pointed to Zimbra

1/ At the Zimbra i was configure forwarding in each mailbox for forward message from Zimbra to mailbox in Office 365 -> Its Worked
2/ In the Office 365 i was created a internal relay connector to Zimbra server -> Stuck.

So the issue is when sender from Office 365 to sending a message to recipient in Zimbra. The message status was delivered but when i trace message in Office 365, but after that i receive that
" mail.mydomain.com rejected your message to the following email addresses:

recipient@mydomain.com (recipient@mydomain.com)
Your message was rejected by the recipient email server. Please check the recipient's email address and try resending your message, or contact the recipient directly. For more tips to help resolve this issue, see DSN code 5.1.0 in Exchange Online - Office 365. If the problem continues, contact your email admin.

mail.mydomain.com gave this error:
<sender@mydomain.com>: Sender address rejected: mydomain.com"

So guys please help me to resolve this issue. I don't know why Zimbra is rejected the message. Should i do any action ? Thanks a lot ^^