Outlook.com SMTP Settings

SMTP is what your mail client uses to send. It is configured separately from the incoming connection, which is why an account can receive perfectly while every outgoing message sits in the outbox — only one half is wrong, and it is usually this one.

For Outlook.com there is a complication worth knowing before you start: the settings below are necessary but often not sufficient. How your client signs in matters more than what you type into it.

Outlook.com SMTP settings

These apply to @outlook.com, @hotmail.com, @live.com and @msn.com addresses — all the same service.

If your client has an "outgoing server requires authentication" checkbox, tick it. Older software sometimes leaves it off by default, on the long-dead assumption that anyone on the network can be trusted.

For incoming mail, see our IMAP settings guide.

Start here: your password may not be the problem

This is the most common reason Outlook.com sending fails, and no amount of retyping will fix it.

Microsoft retired basic authentication for Outlook.com — the arrangement where an application connects with just a username and password. Clients must now sign in through Microsoft itself, which issues a limited token and never handles your password directly.

The symptoms are distinctive:

• The error 535 5.7.139.

• A password prompt that reappears no matter how carefully you type.

• Everything worked for years and then stopped without you changing anything.

The test is simple. When you add the account, a compatible client opens a Microsoft sign-in page in a browser window. One that just asks for your password in its own dialogue box is using the retired method and will be refused.

Current versions of Outlook, Apple Mail, Thunderbird and the major phone apps all handle this correctly. If yours does not, update it or move to one that does. Our Outlook security guide covers this in more depth, including what happened to app passwords.

Choosing the port

Port 587 with STARTTLS is the setting to use. The connection opens unencrypted and upgrades to TLS before any credentials are exchanged, so nothing sensitive crosses the network in the clear.

Port 465 with SSL/TLS encrypts from the first byte and is equally secure in principle, though 587 is the documented setting for Outlook.com.

Port 25 is not for mail clients. It exists for server-to-server delivery, and virtually every home broadband provider, mobile network and hotel connection blocks it to limit spam. Any guide still recommending port 25 for a desktop client is out of date.

Work and school accounts are different

If your address was issued by an employer or school, you are probably on Microsoft 365 rather than consumer Outlook.com. That uses smtp.office365.com, and your administrator may have configured or restricted it further.

The two services are frequently confused, and using consumer settings on a business account — or the reverse — produces authentication failures that look like credential problems. Check which you have before troubleshooting anything else.

When mail still will not send

Once you are confident the client signs in through Microsoft, work through these:

• Check the port and encryption pairing. Port 587 expects STARTTLS. Mixing a port from one pairing with encryption from another fails with an error that reads like a network fault.

• Try a mobile hotspot. If a message sends instantly on mobile data but times out on your office or hotel network, the network is blocking outbound mail ports — not your settings. On a corporate network, ask whoever runs it.

• Check the attachment size. A message that will not leave the outbox is sometimes simply too large.

• Check you are not blocked temporarily. Microsoft limits how much an account can send, to contain compromised accounts. Hitting a limit produces a temporary rejection that clears on its own.

Sending limits

Outlook.com caps daily message volume and recipients per message. The limits apply to mail sent through SMTP exactly as they do to mail sent from the website, and a new account is usually held to tighter limits until it has been used normally for a while.

If you regularly need to send in bulk — newsletters, notifications, anything automated — a personal mailbox is the wrong tool. A dedicated sending service is built for it, and will not put your personal account at risk of suspension.