6 Ways to Solve Outlook Error “421 Cannot connect to SMTP server”

Occasionally while trying to send an email in Outlook or while setting up a new mail account in Outlook you may receive the Error “421 Cannot connect to SMTP server”. In this article, we will offer 6 effective ways to fix this issue in a quick time.

6 Ways to Fix Outlook Error “421 Cannot connect to SMTP server”

The MS Outlook application continues to be the mainstay of business communication and collaboration for some of the largest companies in the world. It still retains its position as the most dominant desktop-based mail client in the world. However, for all the accolades it has earned over the years, it continues to be plagued by inane errors. One such error that occasionally crops up is the “421 Cannot connect to SMTP server” error. If this error shows up, you are most likely unable to send emails. We offer here 6 effective ways to solve the issue with minimum effort.

421 Cannot connect to SMTP server

#1. Turn Off any Firewall Applications

For many home and even office users, the Windows firewall provides an effective way of blocking unauthorized access to their systems. However, the application can interfere with the Outlook application causing the error “421 Cannot connect to SMTP server” to show up. Hence, it makes sense to disable the Windows Firewall application or any other firewall application you may be using on your system.

#2. Consider Removing the Email Account and Configure it up Again

At times this specific error can occur while trying to add a new mail account. In such cases, you should remove the said mail account from the system and consider adding it through the Auto Account setup process instead of opting for the manual setup process.

Auto Account Setup

#3. Change the SMTP Settings

In some cases, the default SMTP settings may not work. Typically port 25 is used for SMTP, however in some cases it might be blocked which can lead to this error cropping up. To solve the issue, try out the steps listed below

  • Launch the MS Outlook Application
  • From the File Tab head to the Info tab and click on Account Settings
  • Select the Mail Account which is throwing up the error, then click on Change
  • Next click on More Settings, then from the Internet Email Settings screen, head to the Advanced tab
  • Change the Outgoing Server Port number to 465, then click on OK to save the settings
Internet E-mail Settings(Advanced Settings)

Once done, please open the Internet Email Settings screen again and head to the Outgoing Server settings

  • Mark the checkbox for My Outgoing Server requires authentication
  • Next, choose the option for Use same settings as my incoming mail server and click on OK to save the settings
Internet E-mail Settings(Outgoing Server)

You can get more detailed information at Microsoft support site.

#4. Deal with the Possibility of a Corrupt PST File

In some cases, a corrupted PST data file can also lead to the “421 Cannot connect to SMTP server” cropping up. To isolate this cause, try to repair the PST file using a top-notch recovery application like DataNumen Outlook Repair. Once you have transferred the contents to a new PST file, open it up in MS Outlook, and try sending out emails again.

DataNumen Outlook Repair

#5. Turn Off all VPN and Antivirus Applications

Occasionally certain programs like antivirus tools and VPN applications can conflict with the functioning of the Outlook application. To isolate such causes, turn off any antivirus application or VPN tool you are using and try to send emails in Outlook in a normal fashion.

#6. Run System Restore

In some rare cases, even after performing all the steps listed above, you may still experience the “421 Cannot connect to SMTP server” error. In such cases, you should consider running the System Restore feature in Windows to bring the system back to a previous date when Outlook was working in perfect shape.

Windows System Restore

You can get more detailed information at Microsoft support site.

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