Personal Outlook.com accounts
The outgoing server is smtp-mail.outlook.com on port 587 with STARTTLS, documented in Microsoft's settings reference.
Outlook.com outgoing server
personal accountsThe username is the full email address including the domain, whether that is outlook.com, hotmail.com or live.com. A local part alone is rejected.
Accounts with two-step verification enabled require an app password rather than the account password. Microsoft generates these in account security settings, and each is scoped to the client it was created for.
Microsoft 365 work and school accounts
The outgoing server is smtp.office365.com on port 587 with STARTTLS at TLS 1.3 or 1.2, documented in Microsoft's guide for devices and applications.
Two account-level requirements sit outside the connection settings. SMTP AUTH must be enabled for the specific mailbox, and Microsoft disables it by default for organisations created after January 2020. It is enabled per mailbox in the admin centre, described under authenticated client SMTP submission.
Authentication method is the larger constraint. Microsoft has scheduled basic authentication for client SMTP submission for deprecation and recommends OAuth, with High Volume Email and Azure Communication Services as the documented alternatives for applications.
Port 465 is not supported on this path. Microsoft's guidance is explicit that a device defaulting to 465 does not support the TLS versions required for client submission.
Sending limits
Client submission carries throttling sized for a person rather than an application. Microsoft documents 10,000 recipients per day and 30 messages per minute on this path.
Exceeding those limits results in the account being throttled rather than a clear error at send time, which presents as intermittent failures that are hard to attribute.
The mailbox is also the sending identity. Mail sent this way authenticates as an individual user, so it carries that mailbox's reputation and its address appears as the sender.
Other Microsoft sending methods
Microsoft documents two alternatives to client submission, and they suit different cases.
SMTP relay uses the tenant's MX endpoint on port 25 with an inbound connector authenticating by certificate or static IP. It does not require a licensed mailbox and carries higher limits, and it cannot be used from a third-party hosted service.
Direct Send also uses the MX endpoint on port 25, unauthenticated, and can only deliver to recipients inside your own organisation. Mail to external addresses is rejected.
For application mail at volume, a submission service authenticating as your own verified sending domain avoids all three constraints, giving per-message delivery events, bounce classification and suppression handling that mailbox submission does not provide.
George, our CEO, is blunt about the bar that path carries: authentication must be SPF, DKIM and DMARC aligned with the policy at quarantine or stronger, which Outlook requires of senders above 5,000 messages a day.
Switching providers costs you a settling period
One practical consequence of changing where your mail comes from is that the large receivers need time to adjust. Moving between providers means Gmail and Outlook take a while to get used to your mail arriving from somewhere new, and delivery during that window is worse than it was before or after.
George's read is that the adjustment is short, usually a message or two rather than a long degradation, but it is enough that an A/B comparison run immediately after a switch will not be meaningful. Give it a few sends before drawing conclusions about the new provider.
We record the email stack behind every customer domain as part of onboarding, and Microsoft is a steady share of them, so this settling period is something we plan for rather than discover.
The tenant question sits underneath all of this. A Microsoft tenant provisioned for staff correspondence is not sized for application mail, and running both through it means an application send can throttle the organisation's own email. Separating the sending identity before volume forces the issue is cheaper than untangling it afterwards.