Fix Gmail Send Mail As Outlook SMTP Authentication Error
Question details
The user is unable to add an Outlook or Microsoft 365 account as a "Send mail as" alias in Gmail and receives an SMTP authentication error.
- Product
- Microsoft 365 / Gmail
- Device & OS
- not provided
- Scenario
- Setting up a Microsoft 365 work account as an alias in Gmail to use with a mail-merge plug-in for Google Sheets.
- Observed behavior
- Gmail rejects the connection and returns an SMTP authentication error, occasionally including a "421 Missed upload deadline" message.
Ensure you have the correct Microsoft 365 email address and password, and check if your organization's IT administrator has disabled basic SMTP authentication for your account.
Verify and Update SMTP Server Settings in Gmail
Use this solution to ensure that Gmail is attempting to connect to the correct Microsoft 365 server with the required encryption and port settings.
Incorrect server configurations are the most common cause of SMTP authentication failures. Microsoft 365 requires specific port and encryption settings that Gmail might not auto-fill correctly.
Log in to your Gmail account, click the gear icon in the top right corner, and select 'See all settings'.
Click on the 'Accounts and Import' tab and locate the 'Send mail as' section. Click 'Add another email address'.
In the pop-up window, enter the SMTP Server as 'smtp.office365.com'. Change the Port to 587.
Enter your full Microsoft 365 email address as the Username and input your password. Select 'Secured connection using TLS' and click 'Add Account'.
Enable Authenticated SMTP in Microsoft 365 Admin Center
Use this solution if your organization has disabled basic SMTP authentication, which prevents Gmail from connecting to the Exchange server.
Try WPS Office for Seamless Mail Merge and Document Management
If configuring complex SMTP settings between Microsoft 365 and Google Workspace is slowing you down, consider switching to WPS Office. It provides a lightweight, highly compatible environment for managing spreadsheets and executing mail merges without the hassle of cross-platform email server conflicts.
- 1. Download WPS Office: Visit the official WPS website and click the download button for your operating system.
- 2. Install the Software: Run the downloaded installer and follow the on-screen instructions to complete the setup.
- 3. Open WPS Spreadsheet: Launch WPS Office and open WPS Spreadsheet to easily manage your data and perform alternative mail merge workflows.
Frequently Asked Questions
Why does Gmail show a '421 Missed upload deadline' error?
This error typically indicates a network timeout or a connection breakdown between Google's servers and Microsoft's Exchange servers. It often occurs if the wrong SMTP port is used or if Microsoft is rejecting the authentication attempt.
Can I use a Google Sheets mail-merge add-on with an Outlook address?
Yes, but the Outlook address must be successfully linked to Gmail as a 'Send mail as' alias first. This requires your Microsoft 365 account to have SMTP AUTH enabled.
What are the correct SMTP settings for an Outlook or Microsoft 365 account?
The correct server is smtp.office365.com, the port should be set to 587, and the connection must use TLS encryption.
Why won't my regular Microsoft 365 password work in Gmail's SMTP setup?
If your Microsoft 365 account requires Multi-Factor Authentication (MFA) or Modern Authentication, Gmail's basic SMTP setup cannot process the prompt. You will need to generate an App Password in your Microsoft account security settings to use instead.
