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SMTP error 10051 – Quick fix

by | Feb 5, 2021

Normally, SMTP error 10051 occurs while sending emails from email clients like Outlook, Thunderbird, etc.

Mostly, it occurs due to Network unreachable issue.

Here at Bobcares, we have seen several such SMTP related errors as part of our Server Management Services for web hosts and online service providers.

Today we’ll take a look at how to fix this SMTP error.


SMTP socket error 10051 – Causes and Fixes

Now, let’s see the different causes of this error. Also, let’s see how our Support Engineers fix it.


ISP firewall issues

Many ISP providers prevent their users from accessing “outside” SMTP servers using conventional e-mail programs like Outlook, Outlook Express.

So in order to prevent spamming on the server, the ISP’S block port number 25.


In order to verify if the ISP is blocking SMTP connections (port 25) or not, our Support Engineers ask the client to telnet the SMTP server from the Command prompt (DOS prompt):

1. Start >> Run >> enter “command” >> Click ok
2. Then enter the below command at the dos prompt

telnet 25

If we find any problem with this connectivity at the ISP level, then we suggest our customers contact their ISP to remove the block on that port.


SMTP authentication issue

Another reason could be an improper SMTP authentication from the sender end.

In simple words, the error means that the SMTP server needs authentication. So, If a sender tries to connect to the SMTP server without authentication, the connection will be refused with this error SMTP error 10051.


First, we check the email client settings and ensure that SMTP authentication is enabled in the sender’s email client.

For example, In Outlook, we verify the SMTP server settings by following below steps:

First, we open the email client program (Outlook Express, Outlook) and from this, Tools >> Account Settings >> More settings >> Outgoing server tab >> My Outgoing server (SMTP) requires authentication.


Antivirus on PC causes SMTP error 10051

Some third-party antivirus installed on the user’s PC can block the connection from certain applications.

Also, the established connection can be aborted by software. This can be possibly done by an antivirus program.


In this case, we advise our customers to disable the antivirus software or tweak the antivirus settings.

[Still, not able to resolve this SMTP error? We can help you in fixing it]



In short, SMTP error 10051 occurs due to improper SMTP authentication, ISP firewall issues, Antivirus on PC, and so on. Today, we saw how our Support Engineers resolve this error.


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