Setup
When a local exchange user has an on-premises mailbox that is not in the cloud, the mailbox cannot send emails to 365 cloud-only accounts. Thus, there is an AD account with the local exchange user, which is synced to 365, and the cloud account only exists in 365 and has no local AD account.
Resolution
Local Exchange
Open your local exchange and navigate to:
- Mail Flow
- Accepted Domains
- Find the domain that is having the issue and double-click
- Ensure the dial is on “Internal Relay: Email is delivered to recipients in the Exchange organization or relayed to an email server at another physical or logical location.”
- Hit the “Save” button
365
Login to https://admin.exchange.microsoft.com/ and navigate to:
- Mail flow
- Accepted Domains
- Find the domain in question and click it
- Ensure the dial is on “Authoritative: Email is delivered to email addresses listed for recipients in Microsoft 365 or Office 365 for this domain. Emails for unknown recipients are rejected.”
- Hit the “Save” button
Test
- On-prem Users
- Can send emails to the cloud email address.
- Can send emails to 365 migrated mailboxes.
- Can send emails externally.
- Can receive emails from steps the cloud email address, 365 migrated and external.
- 365 Only Mailbox and 365 migrated mailboxes.
- Perform the same test as above, but for the 365 Mailbox and a 365 migrated mailbox.
