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.