IIS 6 SMTP virtual server · migration
Replacing IIS SMTP? Check its dependencies first.
Microsoft calls the old IIS SMTP virtual server long unsupported. Windows Server 2025 removed the SMTP Server feature. Before selecting a replacement, identify the applications and devices relying on the old relay. SMTPReady is one option—not a general-purpose mail server.
Microsoft source · Last technically verified · Original documentation ↗
Available approaches
Choose a replacement that handles your actual mail flow.
No single replacement wins every situation. In particular, a relay handling multiple sender identities or arbitrary destinations needs more than a simple bridge.
Change each sender
Few senders; they can use a supported Microsoft route directly.
Microsoft 365 client submission, SMTP relay, Direct Send or HVE.
Microsoft 365 connector relay
Your network meets Microsoft’s certificate/static-IP and port 25 requirements.
No extra local SMTPReady host may be needed.
Exchange Edge Transport
Microsoft transport features, multiple identities or broader routing justify the operations.
A fuller mail-transport component, with higher operational complexity.
Postfix or another MTA
You need general SMTP routing, queues or more than one Microsoft sending identity.
Requires ownership of that mail server and its security posture.
Hosted SMTP provider
You can let another provider relay your outbound mail.
Often simpler when local custody and the Microsoft 365 outbound path are not required.
SMTPReady
The old relay serves local SMTP clients and can be mapped to one Microsoft 365 relay mailbox.
Local Windows queue; customer-owned Graph authorization; limited scope.
Before the cutover
Inventory clients before moving the relay.
The change is bounded only if you understand what the old relay currently does.
Clients can continue speaking SMTP. SMTPReady delivers through Microsoft 365, not arbitrary destinations. Hostnames, ports, sender addresses or credentials may also need to change depending on the existing relay.
- 01List every relay consumer
Collect source IPs, application owners, message volume, recipient types and used sender addresses.
- 02Check what IIS actually does
Record relay restrictions, smart-host settings, DNS or hard-coded targets, authentication and any rewrite behavior.
- 03Select the replacement path
If one Microsoft 365 mailbox cannot cover the sender identities and permissions, use a broader alternative.
- 04Prove the replacement before touching clients
Install on a tested Windows host; configure ingress; prove Microsoft send permission and unrelated-mailbox denial.
- 05Move one sender, compare results, then repeat
Keep rollback addresses. Observe Microsoft acceptance and application behavior before retiring IIS SMTP.