Exchange Online has a new domain expiring soon alert, which means that a mail flow insight checks for soon-to-expire domains in the set of accepted domains for a tenant. When the insight detects a domain due to expire within 90, 60, and 30-day windows, it sends a high-severity alert to the tenant administrators to tell them that they should renew the domain(s).
Step by step process to decommission a shared SMTP namespace in an Exchange Server organization.
An NDR 550 5.1.1 User Unknown error may occur in an Exchange Server environment that uses Internal Relay domains.
How to avoid creating an infinite email routing loop in Exchange Server 2007/2010 when configuring Internal Relay domains.
Learn about open relays and how to verify that your Exchange Server is not one of them.