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Fix Firebase SMTP Authentication Failure with smtp.office365.com

Anum Mushtaq Oct 20, 2026 868 views

Question details

The user is unable to send password-reset emails through an iOS application using Firebase Authentication because the SMTP connection to smtp.office365.com fails.

Fix Firebase SMTP Authentication Failure with smtp.office365.com

Product: Firebase Authentication, Microsoft 365
Device & OS: iOS
Scenario: Configuring Firebase to send emails via smtp.office365.com using port 587 and STARTTLS with an Outlook account.
Observed behavior: SMTP authentication continuously fails despite having organization-level SMTP restrictions removed and SMTP AUTH ostensibly enabled.

Before you start

Ensure you have administrative access to both your Firebase console and your Microsoft 365 admin center to verify email templates and individual mailbox configurations.

Solution 1Recommended

Verify Mailbox-Level SMTP AUTH and Security Settings

Ensure that SMTP Authentication is correctly enabled for the specific Microsoft 365 mailbox and that tenant-level security defaults are not blocking basic authentication.

Even if organization-level SMTP restrictions are removed, Microsoft 365 often blocks basic SMTP authentication through global 'Security Defaults' or individual mailbox settings. Firebase requires basic SMTP AUTH to send emails.

Additionally, Microsoft 365 requires TLS 1.2 or higher. Older server environments attempting to connect might be rejected if they default to older TLS versions.

Check Microsoft 365 Admin Center

Log in to the Microsoft 365 admin center, navigate to Users > Active users, and select the user account used for Firebase. Click on the Mail tab and select 'Manage email apps' to ensure 'Authenticated SMTP' is checked.

Verify Tenant Security Defaults

Go to the Azure Active Directory admin center > Properties > Manage Security defaults. If enabled, this blocks basic authentication (including SMTP AUTH) globally. You may need to disable it and use Conditional Access policies instead, or use an App Password.

Confirm TLS 1.2 Support

Verify that the backend infrastructure handling the Firebase trigger supports and forces TLS 1.2 or later, as Microsoft 365 no longer accepts TLS 1.0 or 1.1 connections.

Verify Mailbox-Level SMTP AUTH and Security Settings

App Passwords: If multi-factor authentication (MFA) is enabled for the Outlook account, you must generate and use an App Password in your Firebase SMTP configuration rather than the standard account password.

Solution 2

Validate Firebase Email Template and Port Configuration

Confirm that Firebase is correctly configured to use port 587 and that the password-reset template matches the authenticated sender address.

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Frequently Asked Questions

Why does Microsoft 365 block SMTP authentication by default?

Microsoft 365 blocks legacy basic authentication (which includes traditional SMTP AUTH) by default to protect against brute-force attacks. This is usually enforced via Security Defaults in Azure Active Directory.

Do I need an app password for Firebase SMTP with Office 365?

Yes, if the Microsoft 365 account has Multi-Factor Authentication (MFA) or two-step verification enabled, your standard password will fail. You must generate an App Password from your Microsoft account security settings and use it in Firebase.

How can I verify if port 587 is accessible from Firebase?

While Firebase handles the outbound connection, ensure no third-party firewalls or routing rules on your custom domain are blocking outbound traffic on port 587. Using STARTTLS on port 587 is the standard requirement for connecting to smtp.office365.com.

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