Settings guide
Office 365 SMTP Settings
Office 365 (Microsoft 365) SMTP settings: server smtp.office365.com, port 587 with STARTTLS, and the full address of a licensed mailbox with SMTP AUTH enabled. Below: limits, the three ways to send, and fixes for common errors.
- SMTP server
- smtp.office365.com
- Port
- 587
- Encryption
- STARTTLS (TLS 1.2+)
- Username
- [email protected]
- Sign-in
- OAuth 2.0
- SMTP AUTH
- Enabled for mailbox
IMAP (incoming): outlook.office365.com, port 993, SSL/TLS
Settings
SMTP settings for Office 365 and Microsoft 365
These are the settings for SMTP AUTH client submission, the method almost every app, CRM and outreach tool uses.
| Setting | Value |
|---|---|
| Server / smart host | smtp.office365.com |
| Port | 587 (recommended) or 25 |
| Encryption | STARTTLS, TLS 1.2 or 1.3 |
| Authentication | Required. OAuth 2.0 recommended. |
| Username | Full email address of a licensed mailbox |
| From address | The same mailbox, or one it has Send As permission for |
| IMAP (incoming) | outlook.office365.com |
| IMAP port | 993, SSL/TLS |
Port 465 won't work
Personal Outlook.com or Hotmail account? That uses a different server, covered in our Gmail and Outlook SMTP settings. Not sure why there are separate SMTP and IMAP servers? See IMAP vs SMTP.
Methods
Three ways to send through Microsoft 365
SMTP AUTH is the default choice. Direct send and SMTP relay exist for printers and apps that can't sign in.
| SMTP AUTH client submission | Direct send | SMTP relay (connector) | |
|---|---|---|---|
| Server | smtp.office365.com | yourdomain-com.mail.protection.outlook.com | yourdomain-com.mail.protection.outlook.com |
| Port | 587 or 25 | 25 | 25 |
| TLS | Required | Optional | Optional (needed for certificate auth) |
| Authentication | Mailbox sign-in | None | Static IP or TLS certificate on an inbound connector |
| Send outside your organization? | Yes | No, internal only | Yes |
| Needs a licensed mailbox? | Yes | No | No |
| Limits | 10,000 recipients/day, 30 messages/min | Standard anonymous-mail throttling | “Reasonable limits”; no spam or bulk |
Microsoft also offers High Volume Email for large internal-only sends and recommends Azure Communication Services for high-volume email to external recipients.
Limits
Office 365 sending limits
These limits apply per mailbox on Microsoft 365 and Office 365 business and enterprise plans. They are ceilings, not targets.
| Limit | Value |
|---|---|
| Recipient rate limit | 10,000 recipients per rolling 24 hours |
| Message rate limit | 30 messages per minute |
| Recipients per message | Customizable up to 1,000 |
| Tenant external recipients | Depends on license count; trial tenants capped at 5,000 per day |
| Sending from *.onmicrosoft.com | 100 external recipients per organization per 24 hours |
Daily ceiling per mailbox vs safe cold email volume
Microsoft 365 counts recipients per day; Google Workspace counts messages per day. Cold email figure is the planning number we use per mailbox.
| Microsoft 365 | 10,000 |
|---|---|
| Google Workspace | 2,000 |
| Safe cold email | 30 |
Hit the recipient limit and the mailbox can't send again until its count over the past 24 hours drops below 10,000. For outreach, reputation runs out long before the quota does, which is why teams spread volume across many mailboxes and domains.
Setup
How to set up Office 365 SMTP: enable SMTP AUTH
SMTP AUTH is off for organizations created after January 2020. Turn it on only for the mailboxes that need it.
Step 1
Open the user
Microsoft 365 admin center › Users › Active users › select the user.
Step 2
Mail tab
Open Mail, then Manage email apps.
Step 3
Check the box
Tick Authenticated SMTP.
Step 4
Save
Select Save changes.
Exchange Online PowerShell
# One mailbox (overrides the organization setting)
Set-CASMailbox -Identity [email protected] -SmtpClientAuthenticationDisabled $false
# Whole organization
Set-TransportConfig -SmtpClientAuthenticationDisabled $falseIf security defaults are on in Microsoft Entra ID, SMTP AUTH is already disabled, and an authentication policy that disables basic auth for SMTP blocks password sign-ins even with both settings on.
Basic authentication is being retired
Sending cold email from Microsoft 365?
Troubleshooting
Office 365 SMTP errors and fixes
| Error | Cause | Fix |
|---|---|---|
535 5.7.3 Authentication unsuccessful | Wrong credentials, SMTP AUTH off, or MFA, security defaults or Conditional Access blocking the sign-in. | Enable SMTP AUTH for the mailbox and switch the app to OAuth. |
5.7.57 Client not authenticated to send mail | Same causes as above. | Same fixes as above. |
535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Tenant | SMTP AUTH is turned off for the organization. | Enable it per mailbox with Set-CASMailbox. |
5.7.60 Client does not have permissions to send as this sender | The From address differs from the signed-in mailbox. | Grant Send As, or send from the signed-in address. |
| Client was not authenticated to send anonymous mail during MAIL FROM | The app connected to smtp.office365.com without signing in. | Enter mailbox credentials, or use direct send / relay instead. |
550 5.1.8 Bad outbound sender | The mailbox was blocked for sending spam. | Remove it from the restricted users list in the Microsoft Defender portal. |
| Connection timed out | Your network or ISP blocks port 587 or 25. | Open the port, or test with telnet smtp.office365.com 587. |
Mail sends but lands in junk? That's authentication or reputation, not SMTP. Check your domain with the SPF, DKIM and DMARC checker and read the Microsoft 365 DKIM setup section.
Sources: Microsoft: send email from apps and devices · Microsoft: enable or disable SMTP AUTH · Microsoft: fix SMTP issues · Exchange Online limits · Basic auth retirement for SMTP AUTH · POP3 and IMAP4 in Exchange Online · Google Workspace sending limits
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Or see pricing →Frequently asked questions
What are the Office 365 SMTP settings?
Server smtp.office365.com, port 587 with STARTTLS (TLS 1.2 or 1.3), authentication required, username set to the full email address of a licensed Microsoft 365 mailbox. Microsoft recommends signing in with OAuth rather than a password.
What port does Office 365 SMTP use?
Port 587 is recommended; port 25 also works for authenticated submission. Port 465 doesn't: Microsoft says an app that defaults to 465 doesn't support the TLS versions SMTP AUTH requires.
How do I enable SMTP AUTH in Office 365?
In the Microsoft 365 admin center go to Users > Active users, select the user, open Mail > Manage email apps and check Authenticated SMTP. Or run Set-CASMailbox -Identity [email protected] -SmtpClientAuthenticationDisabled $false in Exchange Online PowerShell.
Is basic authentication still supported for Office 365 SMTP?
For now. Microsoft's January 2026 update says SMTP AUTH basic authentication stays unchanged through December 2026, is disabled by default for existing tenants at the end of December 2026, and is unavailable by default for tenants created after that. A final removal date is due in the second half of 2027. Move apps to OAuth.
How many emails can I send through Office 365 SMTP?
Each mailbox can send to 10,000 recipients per 24 hours, at up to 30 messages per minute, with at most 1,000 recipients per message. Tenants also have a limit on external recipients per day that depends on how many licenses they hold.
Why do I get "5.7.57 Client not authenticated to send mail"?
The app didn't sign in successfully. Check that SMTP AUTH is enabled for the mailbox, that security defaults or a Conditional Access policy isn't blocking legacy authentication, and that the username and password or OAuth token are correct.