How can I resolve issues with email delivery delays in Exchange?

Verify DNS Configuration

Ensure that the DNS records for the domain mail is hosted on are configured correctly to send and receive mail.
Check for Emails Being Sent to an External Recipient

If an email is sent to an external recipient, check the external reputation of the domain name to verify that the reputation of the IP address sending the email is good.
Check Inbound Connectors

Verify that the inbound connectors are configured properly to allow emails from external domains.
Review Send and Receive Connectors

Inspect the send and receive connectors within Exchange for any misconfigured settings.
Analyze Exchange Traffic Logs

Analyze the Exchange traffic log data to identify any possible root causes of the issue.