You have a hybrid deployment that contains a Microsoft exchange Online tenant and anon premises Exchange Server 2019 server named Server1. Alt users use an email address suffix of @contoso.com.

On Server1, you create a new mailbox that uses an email address of user1@contoso.com

Users hosted in Exchange Online report that they receive a non-delivery report (NDR) When they attempt to send email messages to user1@contoso.com. The NDR contains the following text: "User1 wasn't found at contoso.com."

You verify that the Exchange Online users can send email successfully to the other mailboxes hosted on Server1. Users hosted on Server1 can send email to user1@contoso.com successfully.

You need to identify what causes the email delivery to fail. What should you use?