Microsoft 365 · device and application mail
Which Microsoft 365 SMTP route actually fits?
Choose from the methods Microsoft documents before adding another relay. The decisive questions are whether you need external recipients, what authentication your sender supports, and what the network permits.
Microsoft source · Last technically verified · Original documentation ↗
Documented methods
Start with the constraints, not the product name.
Methods below have different recipient, identity and network requirements. Microsoft’s linked documentation controls if the service changes.
Client SMTP submission (OAuth)
Internal + external
Modern OAuth and TLS 1.2+ on sender; licensed Microsoft mailbox; SMTP AUTH permitted.
Best when the device/application already supports OAuth.
Microsoft 365 SMTP relay
Internal + external
Inbound connector, TLS certificate or static public IP visible to Microsoft; outbound TCP 25.
Often the simplest route when connector/network requirements are met.
Direct Send
Internal only
External-mail-server responsibilities and port 25; Microsoft advises advanced administrators only.
Cannot relay mail to external recipients through Microsoft 365.
High Volume Email
Internal only
HVE account, supported endpoint/authentication and TLS; distinct service constraints.
Good for eligible high-volume internal messaging; not general external relay.
SMTPReady
Internal + external subject to Microsoft sending policy
One local Windows host, one Microsoft 365 tenant and one relay mailbox; customer-owned Graph access.
Only when a local SMTP bridge is required and a native option does not fit.
A different outbound service
Sometimes you do not need Microsoft 365 as the relay.
If a different provider is acceptable, consider a hosted SMTP service or Azure Communication Services Email. If you need general local mail-server behavior, use a full MTA rather than treating SMTPReady as one.
Another hosted relay
Less local infrastructure, but mail follows the provider’s service path, billing and retention terms.
General local SMTP server
Better for multiple routes, identities or broad SMTP/MTA semantics. You own its security and administration.