Start with the requirement
Choose based on sender identity, recipient scope, client capability, network constraints, volume, and who will operate the solution.
Direct Microsoft Graph
The preferred path for new software that can implement OAuth and an HTTP API. It avoids an SMTP bridge but requires development and token handling.
Microsoft client SMTP submission
Fits clients that support OAuth for SMTP. Microsoft documents port 587, TLS 1.2 or later, a licensed mailbox, and mailbox sending limits. Older devices often cannot perform the OAuth flow.
Microsoft 365 SMTP relay
Fits organizations able to operate an inbound connector authenticated by certificate or a static unshared public IP. It uses port 25 and supports broader sender scenarios.
Direct Send and High Volume Email
Direct Send is limited to recipients in the Microsoft 365 organization. High Volume Email is designed for high-volume internal mail. Consult current Microsoft documentation because these services evolve.
smtpify365
Fits an SMTP-only client that supports STARTTLS and authentication, needs external recipients, and should send as one connected mailbox without storing that mailbox password.