Complete troubleshooting guide to fix Office 365 SMTP errors, authentication failures, TLS issues, and sending problems.
Start Fixing →Office 365 SMTP issues usually appear after account security changes, migration updates, credential mismatches, authentication restrictions, or incorrect application configuration.
Most applications should use authenticated SMTP submission instead of open relay configuration.
Open Microsoft 365 admin settings and confirm SMTP AUTH is enabled for the mailbox.
Use the complete email address instead of aliases.
Use port 587 with TLS.
Older SMTP implementations may fail after modern authentication changes.
Testing isolates application configuration issues.
Corporate firewalls may block SMTP traffic.
Update passwords and refresh stored credentials.
Credentials or SMTP AUTH issue.
Mailbox lacks relay permissions.
Connection interrupted.
Encryption mismatch.
Review DNS configuration, SPF records, mailbox permissions, conditional access policies, security defaults, and application integration settings.
If mail originates from websites, scanners, printers, CRM systems, or contact forms, validate their SMTP transport configuration independently.
Monitoring logs can help identify intermittent failures and authentication changes.
Yes, authenticated SMTP remains available depending on tenant settings.
TLS.
Yes. Contact forms frequently depend on SMTP delivery.
That is the commonly recommended submission port.
If SMTP email settings are not working in Office 365, most issues can be traced to authentication, ports, TLS configuration, or account security policies. Confirm settings carefully and test changes methodically to restore reliable email delivery.