Fix SMTP error 421, 451

Well, you might be receiving an error message while sending an email to your recipient in the form of a three-digit number, each containing a detail about the server or an answer to a command. First, try restarting the mail server but if it keeps coming up follow the steps below. So let’s see how to fix SMTP error 421, 451:

421 SMTP Server error: too many concurrent sessions

SMTP error 421 denotes either a temporary problem on the mail server or a problem with the recipient's email account.

If you reached a limit (restriction) on your mail account you may get a 421 error as some mail providers might return this error.

Depending on the cause we will see how to fix SMTP error 421. You need to try each below solution until it’s fixed.

Solution1: Fix the Outlook Configuration Issues

If this issue is caused right after the configuration of the outlook account, then the error is occurred due to a configuration mistake. The invalid port number setting or mail server name will show “421 cannot connect to SMTP Server” error while sending or receiving an email so remove the email account and add the account again.

To add the configuration correctly follow the below steps:
Step 1 - Open Outlook account and go to File > Account Setting, Select the email account and click Remove button.
Step 2- Once the account is removed, click the New button then select the Email account and enter the required information. Click Next and wait for the settings to be configured and check the test mail sent.
Step 3- If the test email delivered successfully, then your account is configured properly. In case, if you face the same error then follow Solution2.

Solution2: Change the SMTP port number

When you enter a wrong SMTP port number you will end up receiving an SMTP 421 error. You can check by changing the SMTP port number from Port 25 to 465 and see if the normal functionality resumes. 
For this you need to go to File > Account Settings, then choose the email account that’s affected and click on the Change button.
In the Change Account window, select More Settings > Advanced tab and now set to 465 in modify the Outgoing server (SMTP).
Ensure ‘Use the following type of encrypted connection: is set to SSL ‘.

Solution3: Check 3rd party firewall interference

Though the firewall protects us against malicious attacks, it can block certain IP ranges, so let’s test by disabling 3rd party firewall and send a test mail.
For that Open the Control Panel > under Programs and Features section choose your 3rd -party antivirus and uninstall it. Then check by restarting your PC.
You can install the firewall from the official site and re-check the error by sending a test mail.

Solution4: Check VPN interference

Try sending a test mail once after you disable your VPN connection, if the mail is sent successfully, then the VPN will be the cause of the error. If the error persists you might probably need to contact the VPN network support and ask to whitelist the email server.

SMTP error 451 Exchange Server Error

Next, we will see how to fix SMTP error 451. When you face issues as mail limits exceeded, DNS errors, unreachable mail servers, etc., the Email Relay environment will get affected due to a temporary exchange server. This 451 server error can be due to a discrepancy on Sender or Receiver Exchange server. Let’s see how to resolve the 451 server error.

Step to resolve Exchange Server Error 451

Inspect Queue Viewer

• Open Exchange Management Console > Click Edge Transport server.
• Then go to toolbox > Queue Viewer.
• Select Mail Flow tools > Queue Viewer tool.
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Verify DNS Configuration

• Login to the Edge Transport server locally and use Console Switch, if you are accessing “Remote Desktop Connection”.
• Next, open Exchange Management Console > Edge Transport Server > properties.
• Then go to Internal DNS Lookups and check whether it is in Default Configuration Setting. If not, set it to Default.
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Implementing Internal and External Lookups

1. Internal Lookups
When you have multiple network adapters, you need to go with an Internal Network Card and then choose the card to access the DNS setting. If any misconfiguration occurs, it will allow all the IP addresses to modify.
If there is no IP address detected, then the NIC card might not be configured with DNS server entries. Therefore fill the card with appropriate details and confirm the configuration.   

 
2. External Lookups
If you are using a single network card with public DNS, modification of configuration may affect external resolution and it might stop the email flow.

Command Line tools to test DNS Server

NSLOOKUP – To get DNS records that hold Server & IP address Information.
TELNET – To test ports.


Locate IP Address of the Server

• First, type “nslookup” to find a server IP address and click to open the NSLOOKUP tool.
• Next, type set type=mx and press enter.
• Finally, enter the domain name you wish to extract the MX record and hit enter.
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Check out SMTP connection Via TELNET command

• Open Command Prompt (cmd) and enter telnet.
• Enter set logfile (location:/filename)
• Next, type open mail.abc.com 25 and press enter.
• Now type the command EHLO contoso.com and execute it.
• Compose test email message with parameters like from, receive, notify, data and subject.
• After the process termination, it will generate a success or failure confirmation report.
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Hope you would have fixed. If the issue still persists or any other email error Try Expert Solution.
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