smtp.office365.com not working

Check the settings

Server

smtp.office365.com

Port

587 (recommended) or 25

Encryption

STARTTLS, TLS 1.2 or TLS 1.3

Username and password

The sign-in of a licensed mailbox

Limits

10,000 recipients per day. 30 messages per minute.

Port 465 won't do: “If your device or application recommends or defaults to TCP port 465, it doesn't support the required versions of TLS for client SMTP submission in Microsoft 365 or Office 365.”

Source: Microsoft Learn, How to set up a multifunction device or application to send email using Microsoft 365 or Office 365 (read October 30, 2026)

Read the server's reply

Look in the app's log, or on the copier's screen or send history, and match the reply here:

What Microsoft says causes it

• SMTP AUTH is off for the organization or the mailbox. It's disabled for organizations created after January 2020 unless it is enabled per mailbox. Turn it on.
• Password sign-in is blocked: “If security defaults is enabled in your organization, SMTP AUTH is already disabled in Exchange Online. To use SMTP AUTH, you need to disable security defaults.” And: “If your authentication policy disables basic authentication for SMTP, clients cannot use the SMTP AUTH protocol even if you enable the settings outlined in this article.”
• The sign-in is refused because of multifactor authentication on the mailbox, security defaults or a Conditional Access policy that blocks legacy authentication. Microsoft's four checks.
• Old TLS. “Recently, we started rejecting a percentage of connections to smtp.office365.com that uses TLS1.0/1.1 for SMTP AUTH. Your device must support TLS version 1.2 or above.” More.
• A blocked port. “If you can't connect to Microsoft 365 or Office 365, your network firewall or Internet Service Provider (ISP) might have blocked port 587 or 25.”
• The From address isn't the sign-in mailbox (5.7.60). Send As, or the relay route.

Sources: Microsoft Learn, Enable or disable authenticated client SMTP submission (SMTP AUTH) in Exchange Online; Microsoft Learn, How to set up a multifunction device or application to send email using Microsoft 365 or Office 365; Microsoft Learn, Fix issues with printers, scanners, and LOB apps that send email using Microsoft 365 (read October 30, 2026)

For apps and scripts

“SMTP AUTH supports modern authentication (Modern Auth) through OAuth in addition to basic authentication.” For code you control, Microsoft documents SASL XOAUTH2 with an access token issued for the https://outlook.office.com/SMTP.Send permission, so the app never stores a mailbox password.

Off-the-shelf software that only takes a server, a username and a password is in the same spot as an older copier: it needs a route that does the Microsoft sign-in for it.

Sources: Microsoft Learn, Enable or disable authenticated client SMTP submission (SMTP AUTH) in Exchange Online; Microsoft Learn, Authenticate an IMAP, POP or SMTP connection using OAuth (read October 30, 2026)

After December 2026

Microsoft's notice MC786329: “End of December 2026: SMTP AUTH Basic Authentication will be disabled by default for existing tenants.” Admins can still turn it back on for now, and Microsoft will announce the final removal date in the second half of 2027. Routes that don't depend on password sign-in:

• OAuth on the device. SMTP AUTH supports OAuth as well as passwords, and it's the method Microsoft recommends. It works if your copier's maker supports it on your model and firmware: see the guide and your brand's page.
• An SMTP relay connector. The device sends to your MX endpoint on port 25, and Microsoft 365 recognises it by your static IP address or a certificate. Steps.
• Direct Send or High Volume Email, if scans only ever go to people inside your organization. Details.
• A third-party relay, which Microsoft's notice names for anything that must keep a username and password: “if you must use Basic auth to send email with Exchange Online, then you must use one of the alternatives or a 3P solution.”

Sources: Microsoft 365 Message Center notice MC786329 (updated January 27, 2026); Microsoft Learn, Deprecation of Basic authentication in Exchange Online (read October 30, 2026)

Common questions