--- title: "Define a custom SMTP server" slug: "content-define-a-custom-smtp-server" updated: 2026-09-24T14:27:05Z published: 2026-09-24T14:27:05Z canonical: "help.8x8.com/content-define-a-custom-smtp-server" --- > ## Documentation Index > Fetch the complete documentation index at: https://help.8x8.com/llms.txt > Use this file to discover all available pages before exploring further. # Define a custom SMTP server SMTP servers control your contact centre's email delivery process. You may use the 8x8 default **VCC Internal** SMTP Server, or configure a custom server. By default, your tenant outbound email communication is routed via the pre-configured 8x8 Contact Center Internal SMTP Server. 8x8 Contact Center also offers the flexibility to configure a custom SMTP server for better security and HIPAA compliance. Tenants can leverage their own infrastructure and ensure email delivery to the final recipient. 8x8 Contact Center establishes a secure connection with the tenant SMTP server. Using a default server ensures that all outgoing emails originating from email channels or agents will use the default server and a single default account. Using a custom SMTP server, however, allows email channels and agents to use distinct accounts for outgoing emails, as configured individually. To protect transmissions against interception, SMTP servers must be configured with authentication and the STARTTLS protocol. Microsoft no longer supports basic authentication (username and password credentials) for SMTP; therefore, choose Microsoft Exchange as your authentication method when setting up Microsoft 365 mailboxes. Follow the instructions below to configure a custom SMTP server. ## Prerequisites for Connecting a Microsoft 365 Mailbox Ensure the following requirements are met in Microsoft 365 before clicking **Connect**. If any requirement is unfulfilled, sign-in may succeed, but connection tests and outgoing emails will fail. - **Matching Mailbox Primary Email Address and User Principal Name (UPN)**: The mailbox's User Principal Name (UPN) must match its primary email address. During authentication, 8x8 uses the mailbox email address, which Microsoft verifies against the UPN. If they differ, Microsoft will reject the connection with a 535 5.7.3 Authentication unsuccessful error, even if the same mailbox worked with basic authentication before. Resolve this by setting the UPN to match the primary email address via the Microsoft 365 admin center (**Users** > **Active users** > **[selected user]** > **Manage username**), or select a mailbox with matching attributes. - **Enabled Authenticated SMTP**: Enable Authenticated SMTP in the Microsoft 365 admin center under **Users** > **Active users** > **[selected user]** > **Mail** > **Manage email apps**. Additionally, ensure Security defaults in Microsoft Entra is disabled; otherwise, this setting is ignored for the whole tenant. - **User Account Authentication for Shared Mailboxes**: Direct sign-in is not supported for shared mailboxes. Connect with a licensed user that has **Send As** permission on the shared mailbox. **To configure a custom SMTP server in** 8x8 Configuration Manager**:** 1. From the **Configuration Menu**, open **Security**. 2. Go to the **SMTP Servers** tab. > [!NOTE] > Note: > > By default, your outbound email communication is routed through the pre-configured 8x8 Contact Center Internal SMTP Server. 3. Click ![](https://cdn.us.document360.io/e3a59e39-abd3-4423-964c-0a4008dc5673/Images/Documentation/icon_plus_18x18.png?sv=2026-02-06&spr=https&st=2026-11-03T22%3A57%3A23Z&se=2026-11-03T23%3A57%3A23Z&sr=c&sp=r&sig=xfLTpqH3QLRUWy1YD5%2FJvmJvqZWRku34%2BQ%2Bf1Z55G3g%3D)**SMTP Server**. 4. Enter the following information. | SMTP Server Definition | Description | | --- | --- | | Name | A label for the SMTP server, such as [AcmeJets SMTP Server]. | | Default | Select this checkbox to make this SMTP server the default for your tenant. | | Server Name | The name of the SMTP server as registered, such as [smtp.gmail.com]. | | Port | The port number depends on the connection security method you choose. The default port for SMTP server is 25. For STARTTLS, use port 587. For SSL/TLS use oprt 465. | | Connection Security | The method used to encrypt communication: SSL/TLS or STARTTLS. | | Authentication Type | **Basic**: Authenticate with a username and password, using one of the authentication methods below. **Microsoft Exchange**: Use this option for all Microsoft 365 m. ailboxes. To connect a Microsoft account: 1. Click **Connect.** 2. Sign in with a Microsoft 365 account to allow 8x8 to send email on your behalf. 3. This avoids saving credentials, and sets **Server Name**, **Port**, and **Connection Security** for you ([smtp.office365.com](http://smtp.office365.com), 587, STARTTLS). The connected mailbox appears in the **User Name** field.avoid saving credentials. **Server Name**, **Port** and **Connection Security** are set for you ([smtp.office365.com](http://smtp.office365.com), 587, STARTTLS). ![Connect SMTP server using Microsoft Exchange authentication, then choose Use Password to sign in with OAuth](https://cdn.us.document360.io/e3a59e39-abd3-4423-964c-0a4008dc5673/Images/Documentation/smpt-ms-exchange-connect(3).png?sv=2026-02-06&spr=https&st=2026-11-03T22%3A57%3A23Z&se=2026-11-03T23%3A57%3A23Z&sr=c&sp=r&sig=xfLTpqH3QLRUWy1YD5%2FJvmJvqZWRku34%2BQ%2Bf1Z55G3g%3D) *Configure the SMTP Server settings to use Microsoft Exchange authentication, then click Connect and select Use Password to complete Microsoft OAuth sign-in* | | Authentication Method | The method used to encrypt the password: - **Password, transmitted insecurely (PLAIN)**: Sends the password without encryption, using the LOGIN method. - **Encrypted password (CRAM-MD5)**: Sends the password with encryption. > [!NOTE] > Note: > > Applicable exclusively to **Basic** authentication. > > **Microsoft 365** requires OAuth2 and no longer supports **Basic** authentication for SMTP. When setting up Microsoft 365 mailboxes, select **Microsoft Exchange** as the Authentication Type. | | User Name | Usually the email address for the account, for example your Gmail address for Gmail SMTP. When using **Microsoft Exchange**, this field displays the mailbox used during sign-in and cannot be changed. | | Password | The password for the email account associated with this SMTP server. > [!NOTE] > Note: > > This field does not apply when using **Microsoft Exchange** authentication. | | Enter an email address to test the SMPT server configuration | Enter a working email address to send a test email. The test verifies if the SMTP server is configured correctly. | 1. Click **Test** to verify the setup. If the configuration is correct, the system sends a test email to the address you entered. Check your inbox to confirm whether the test message arrived. > [!NOTE] > Note: > > If the test for a **Microsoft Exchange** server does not succeed, see [Troubleshoot Microsoft Exchange authentication](/documentation/docs/content-define-a-custom-smtp-server#troubleshoot-microsoft-exchange-authentication). ![Test SMTP server configuration using Microsoft Exchange authentication in 8x8 Configuration Manager](https://cdn.us.document360.io/e3a59e39-abd3-4423-964c-0a4008dc5673/Images/Documentation/smpt-ms-exchange-connection-test.png?sv=2026-02-06&spr=https&st=2026-11-03T22%3A57%3A23Z&se=2026-11-03T23%3A57%3A23Z&sr=c&sp=r&sig=xfLTpqH3QLRUWy1YD5%2FJvmJvqZWRku34%2BQ%2Bf1Z55G3g%3D) *Send a test email through the configured SMTP server to verify that Microsoft Exchange authentication is working correctly* 2. Select the **Default** checkbox to set this server as the default now. Alternatively, you can designate it as the default server later, **SMTP Servers** list. 3. Click **Save**. ![](https://cdn.us.document360.io/e3a59e39-abd3-4423-964c-0a4008dc5673/Images/Documentation/setup-custom-smtp-server.gif?sv=2026-02-06&spr=https&st=2026-11-03T22%3A57%3A23Z&se=2026-11-03T23%3A57%3A23Z&sr=c&sp=r&sig=xfLTpqH3QLRUWy1YD5%2FJvmJvqZWRku34%2BQ%2Bf1Z55G3g%3D) A check mark in the **Set as default** column identifies the current default server. If your new SMTP server isn't already set as the default, select the check box in the **Set as default** column to make it the default. ![](https://cdn.us.document360.io/e3a59e39-abd3-4423-964c-0a4008dc5673/Images/Documentation/custom-smtp-server-set-default.png?sv=2026-02-06&spr=https&st=2026-11-03T22%3A57%3A23Z&se=2026-11-03T23%3A57%3A23Z&sr=c&sp=r&sig=xfLTpqH3QLRUWy1YD5%2FJvmJvqZWRku34%2BQ%2Bf1Z55G3g%3D) When you change the default server, any email channel or agent that doesn't have an individual configuration overriding the default switches to the new default server. > [!NOTE] > Note: > > You connot delete the default server unless you set another server as default first. To see the channels and users currently using the server, click the number under **Used by Users** or **Used by Channels** lists. > [!NOTE] > Note: > > If the system fails to deliver emails because the remote SMTP server is down or a configuration parameter is invalid, the system tries to determine the reason for failure and retries a few times before sending a failure notification. ## Troubleshoot Microsoft Exchange authentication 8x8 Configuration Manager displays a single generic error for any Microsoft sign-in failure: "The Microsoft Exchange configuration is invalid. Please verify your username and reconnect." As Microsoft is the source of the issue, review the table below to identify and fix the exact underlying cause. | Microsoft response | Cause | Fix | | --- | --- | --- | | 535 5.7.3 Authentication unsuccessful | The mailbox UPN does not match its primary email address. | Set the UPN to the primary email address in Microsoft 365, then click **Connect** again. | | 535 5.7.139 SmtpClientAuthentication is disabled for the Tenant | Authenticated SMTP is off for the mailbox or the tenant. | Enable Authenticated SMTP on the mailbox, turn off Security defaults, then click **Test** again. You do not need to reconnect. |