What causes the “421 Cannot connect to SMTP server” error and a quick work around

A colleague of mine has encounters errors like:

An unknown error has occurred. Account: ‘mail.different.bg’, Server: ‘mail.different.bg’, Protocol: SMTP, Server Response: ‘421 Cannot connect to SMTP server 212.70.124.241 (212.70.124.241:25), connect error 10060’, Port: 25, Secure(SSL): No, Server Error: 421, Error Number: 0x800CCC67

while he was trying to send some emails with his Outlook Express mail client on his desktop computer running Windows XP, since he is not too much computer literate he contacted me for help on what is causing the error and how he can get through the issue and send the prepared emails to the destinations ASAP.

After I have asked him a few questions necessary to better understand the status of the problem and where does it originated I have come to the conclusion that it’s very likely that his outgoing SMTP port (25) outgoing TCP/IP traffic passing through the Internet Service Provider is filtered.
When the 421 Cannot connect to SMTP server problem occured, he was actually in his parents house provided with an internet connection through a BTC ADSL see BTC (Vivacom)’s ADSL page for reference

I have instructed my friend to try connecting to the SMTP (25) port of the questionable email server using window’s telnet client i order to check if my assumption that the outoging SMTP 25 port traffic is filtered.

I instructed him to issue a command like which is so common this days and it’s not news to the Sysadmins out there:

cmd> telnet mail.server.net 25

This prooved my theory that the 421 Cannot connect to SMTP server was caused by a filtered traffic on the outgoing network STMP port (25).

Some Internet Providers out there has that annoying practice of filtering the outgoing SMTP connections, because they couldn’t deal with infected Windows computers who start acting as a SPAM networks in another more clever way, however I should admit this is pretty dumb, since it creates numerous problems to the end user like in this particular case.

The temporary work around for him that I suggested was to use the mail server Webmail Interface before he moves back with his notebook back to his ISP at home which doesn’t include such a foolish way to filter spammers.

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7 Responses to “What causes the “421 Cannot connect to SMTP server” error and a quick work around”

  1. RGeo says:
    Firefox 3.6.13 Firefox 3.6.13 Windows 7 Windows 7
    Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.13) Gecko/20101203 Firefox/3.6.13 ImageShackToolbar/5.2.3

    You can also use the online smtp port 25 test at http://port25.icannotconnect.com to check if outbound port 25 is being filtered

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  2. Wszelkie dodatkowe informacje na stronie internetowej: says:
    Internet Explorer 7.0 Internet Explorer 7.0 Windows XP Windows XP
    Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30)

    A mi siÄ™ bardzo podoba! Ogromny plus za takie przedstawienie tematu. Jestem zdecydowanie po Twojej stronie 😀

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  3. Jean-Claude says:
    Microsoft Edge 14.14393 Microsoft Edge 14.14393 Windows 10 x64 Edition Windows 10 x64 Edition
    Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.79 Safari/537.36 Edge/14.14393

    This is not an ISP filtering issue, as connecting via telnet shows that you have the needed connection.

    If you were unable to connect, then you could call this an ISP issue, otherwise, it’s hard to say that once your connected, and ISP is blocking your connection.

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  4. Sascha Wernicke says:
    Opera 8.50 8.50 Opera 8.50 8.50 Windows XP Windows XP
    Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; en) Opera 8.50

    Krasser Einfall. Aber man muss auch sehen, dass man sich nicht immer auf die Einfachheit der Dinge verlassen darf. Realismus ist oft besser als Wolkenkuckungsheime.

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  5. Error Number 2006 Joomla says:
    WordPress 4.8.4 WordPress 4.8.4
    The Incutio XML-RPC PHP Library -- WordPress/4.8.4

    […] What causes the “421 Cannot connect to SMTP server” error. – A colleague of mine has encounters errors like: An unknown error has occurred. Account: ‘mail.different.bg’, Server: ‘mail.different.bg’, Protocol: SMTP. […]

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  6. A Network Error Occured While Connecting To The Server says:
    WordPress 4.8.4 WordPress 4.8.4
    The Incutio XML-RPC PHP Library -- WordPress/4.8.4

    […] What causes the “421 Cannot connect to SMTP server” error. – A colleague of mine has encounters errors like: An unknown error has occurred. Account: ‘mail.different.bg’, Server: ‘mail.different.bg’, Protocol: SMTP. […]

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  7. Joselyn Huezo says:
    Firefox 3.5.3 Firefox 3.5.3 Windows 7 Windows 7
    Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.1.3) Gecko/20090824 Firefox/3.5.3

    Appreciate it for helping out, superb information.

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