Know which account you have first
This is the step that resolves most failures, because the two are easy to confuse and the error they produce is identical.
The account decides, not the app
read the address, pick the hostA personal account ends outlook.com, hotmail.com, live.com or msn.com, and is documented in Microsoft's settings reference.
A work or school account uses a company domain and is administered through a Microsoft 365 tenant, covered in the device and application guide.
Pointing a personal account at smtp.office365.com fails at authentication, which reads as a wrong password rather than a wrong server.
We record the email stack behind each customer domain at Nitrosend, microsoft included, and the personal-versus-tenant split is the first thing that lookup settles, because the two account types fail the same way with different fixes.
In the Outlook desktop application
Account settings hold the values. From File, open Account Settings, then Account Settings again from the dropdown, and open the account entry to view server details.
Some builds surface this under Server Settings within the account rather than in a properties dialog, and the outgoing server appears there alongside the port and encryption method.
Where the account is an Exchange or Microsoft 365 connection rather than an IMAP or POP one, the desktop client may not display an SMTP host at all. Exchange accounts connect over Microsoft's own protocol rather than SMTP, so there is no outgoing server field to read, and the value has to come from the documentation instead.
In Outlook on the web
Settings, reached through the gear icon, contains the mail configuration. Under Mail, the sync or forwarding section carries a link to view POP, IMAP and SMTP settings.
The web interface changes frequently, and the section has moved between Mail and Accounts across versions. If the link is absent, the published values remain correct.
Administrators can disable this display for a tenant, in which case the values come from the administrator rather than the interface.
The settings themselves
For personal accounts: smtp-mail.outlook.com, port 587, STARTTLS, authentication required, username being the full email address.
For Microsoft 365: smtp.office365.com, port 587, STARTTLS at TLS 1.3 or 1.2, authentication required. Port 465 is not supported on this path, and Microsoft states that a device defaulting to it does not support the required TLS versions.
Two account-level requirements sit outside the values. SMTP AUTH must be enabled on the specific mailbox, and Microsoft disables it by default for tenants created after January 2020. Accounts with two-step verification need an app password rather than the account password.
Basic authentication for client submission is scheduled for deprecation in favour of OAuth, so anything built on username and password against a Microsoft account has a limited life ahead of it.
When the answer will not help
Finding the value configures a client to send as one mailbox, which is the right outcome for a person sending their own mail.
An application is a different case. Client submission is throttled at 10,000 recipients per day and 30 messages per minute, sends as an individual rather than as a domain identity you manage, and reports no delivery events or bounce classification back to the sending system.
For that case Microsoft documents SMTP relay through a connector and High Volume Email, and a submission service authenticating as a verified sending domain supplies the reporting layer none of those provide.
Above that scale the receiver rules take over from the client settings. George, our CEO, keeps the current bar written down in his Email Marketing Bible. SPF, DKIM and DMARC aligned at p=quarantine or stronger, which Outlook now requires of anyone sending five thousand a day or more and enforces with a 550 on non-compliant bulk.