SMTP Authentication is a technique that verifies the email sender’s identity to prevent unauthorized access and minimize phishing or spam. Also known as SMTP AUTH, it helps protect your mail server from unauthorized use. When it is enabled, email senders from your mail server must authenticate and prove that the account is valid. If the authentication fails, the server rejects their request.

This article explains in detail how to enable or disable SMTP authentication in Microsoft 365 Exchange Online. To begin, here is a quick overview of this protocol.

What Is SMTP Authentication

SMTP Authentication is a security check that protects an email program from unauthorized access and spam. SMTP, or Simple Mail Transfer Protocol, is an internet standard communication protocol to enable email message transmission between mail servers. It sends, receives, and relays emails across networks using a client-server model. Think of it like a courier service that plans the best route, handles detours, and ensures your package arrives safely.

SMTP AUTH maximizes the SMTP service provider’s scalability and features, and protects your email and account from spam and unauthorized use.

Simply put, SMTP Authentication is an email communication protocol to ensure only authorized users can use an SMTP server to send emails. It safeguards the SMTP server and prevents unauthorized users from exploiting the server to send malicious or spam emails. Through authentication, i.e., verification via credentials such as a username and password, it checks the sender’s identity before the server processes the email and relays it.

SMTP Authentication methods can be of different types, such as PLAIN, LOGIN, CRAM-MD5, and OAuth 2.0, to ensure trustworthy and reliable email communications.

SMTP Authentication types

SMTP AUTH can be:

• PLAIN: Plain text credential transmission; use only if the connection is encrypted.
• LOGIN: Unlike PLAIN, LOGIN sends your credentials (username and password) in separate steps; use with TLS encryption.
• CRAM-MD5: Keeps your credentials protected during transmission through an encryption layer.
• OAuth 2.0: Instead of your password, OAuth 2.0 uses tokens for extra security.

Understanding the above authentication methods helps you choose the best option for your SMTP Authentication setup.

How to Enable or Disable SMTP Authentication

You can use Microsoft 365 Admin Center or Windows PowerShell to enable/disable SMTP Authentication in Microsoft Office 365 Exchange Online. Admin Center enables this feature in only one mailbox at a time; PowerShell enables it for multiple mailboxes. Below are all the methods explained in detail.

Method 1: Use Office 365 Admin Center to enable or disable SMTP Authentication

To enable or disable SMTP Authentication in Microsoft 365 Exchange Online on individual mailboxes:

• Navigate to the Microsoft 365 Admin Center.
• Sign in to Microsoft 365 Admin Center with your account.
• Click the menu icon.
• Click Users.
• Next, click Active Users.
• Find and click the mailbox you need to modify. To enable SMTP Authentication, this user must have a valid Exchange Online license.
• Click the Mail tab

• Next, click Manage email apps.
• Check the Authenticated SMTP box to enable SMTP AUTH. Unchecking this box disables the authentication.
• Lastly, click Save Changes to complete the process.

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Method 2: Enable or Disable SMTP AUTH using Windows PowerShell

To enable Office 365 SMTP Authentication or disable it for individual or multiple users with Windows PowerShell:

• Click the Start button.
• In the search bar, type PowerShell.
• Click Run as administrator in the PowerShell details pane.

• Click Yes when a User Account Control prompt appears.
• In Administrator Windows PowerShell, copy and paste the command: exe -ExecutionPolicy RemoteSigned.
• Press Enter
• Copy and paste the command: Install-Module ExchangeOnlineManagement.
• Press Enter
• To load the ExchangeOnlineManagement PowerShell module, copy and paste the command Import-Module ExchangeOnlineManagement and press
• Run the command Connect-ExchangeOnline -UserPrincipalName (UPN) and press Enter to connect to Exchange Online. In this command, replace UPN with your Office 365 login account using the format UserName@DomainName.com.
• Enter the password for your account and click Sign in.
• After signing in to Exchange Online PowerShell, check the SMTP Authentication status in your Office 365 tenant. To check the status, run the command below in Administrator Windows PowerShell. If the command displays True, SMTP is disabled. If the command displays False, SMTP is enabled.

Get-TransportConfig | Format-List SmtpClientAuthenticationDisabled

• If SMTP is disabled, run the command below to enable it tenant-wide (globally).

Set-TransportConfig -SmtpClientAuthenticationDisabled $false

• Change $false to $true in the previous command if you want to disable the SMTP AUTH protocol Office 365 tenant-wide.
• To enable SMTP Authentication for a single Office 365 Mailbox with PowerShell, run the following command. Remember to change UPN in the command below to the mailbox for which you want to enable SMTP AUTH in the format UserName@DomainName.com.

Set-CASMailbox -Identity (UPN)

• To enable SMTP Authentication for multiple Office 365 mailboxes with PowerShell, run the command below.

Get-CASMailbox -ResultSize unlimited | Where-Object {$_.SmtpClientAuthenticationDisabled -eq $true} | ForEach-Object {Set-CASMailbox -Identity $_.PrimarySmtpAddress -SmtpClientAuthenticationDisabled $false}

Below, we answer some common questions about enabling and disabling SMTP Office 365 authentication to clear up persistent confusion.

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Frequently Asked Questions

Q1. How do I enable SMTP Authentication?

To enable SMTP Authentication, follow the path: Microsoft 365 Admin Center > Menu icon > Users > Active Users > Mailbox > Mail > Manage email apps > Check the Authenticated SMTP checkbox > Save Changes. To enable SMTP AUTH for multiple mailboxes, use Windows PowerShell, as explained in the guide above.

Q2. How do I check whether SMTP is working?

Run the Get-TransportConfig | Format-List SmtpClientAuthenticationDisabled command in Administrator Windows PowerShell to check if SMTP AUTH is working. If the command displays True, SMTP is disabled. Otherwise, it displays False.

Q3. How does SMTP Authentication work?

SMTP AUTH works in a 4-step process. First, the sender’s email client initiates a connection with the SMTP server. The server then sends an authentication request; the server verifies the credentials (username and password) and ensures they match the ones in its database; lastly, the server allows the email to be sent.

Q4. Why is SMTP Authentication Important?

SMTP Authentication is essential to protect the SMTP server from unauthorized access, improve deliverability, ensure compliance with standards, and preserve sender reputation.

Hope this helps. If you have any questions or concerns about SMTP Authentication, leave us a comment.