Troubleshoot mail flow in Exchange Online
Can't send or receive email? Microsoft 365 and Office 365 for business offer admins several ways to troubleshoot. We recommend using the automated solutions because they are typically easier and faster than manual troubleshooting.
For instructions about troubleshooting options, see Find and fix email delivery issues as a Microsoft 365 or Office 365 for business admin.
Troubleshoot mail flow caused by connectors
To validate and troubleshoot mail flow from Microsoft 365 or Office 365 to the email servers in your on-premises organization (also called the on-premises server), validate your connectors. You can set up and validate connectors on the Connectors page in the Exchange admin center (EAC). The built-in validation tests that your mail flow from Microsoft 365 or Office 365 reaches:
Your organization's email server
A partner organization.
For more information, see Validate connectors.
Troubleshoot mail flow issues caused by incorrect SPF records or MX records
How Microsoft 365 uses Sender Policy Framework (SPF) to prevent spoofing gives tips on how to fix several SPF record errors. The beginning of that article also provides an explanation of what SPF records are and how Microsoft 365 and Office 365 use them to prevent spoofing.
Mail flow issues can also happen when your MX record is not setup correctly. To verify your MX record, see Find and fix issues after adding your domain or DNS records.
For more information
Mail flow best practices for Exchange Online and Microsoft 365 or Office 365 (overview)