I got this error while trying to send out mail using my MTA which in my case was sendmail.
Reason:
If you receive this error then it means that your mail server is unable to connect to your smtp provider most probably due to firewall. So check your firewall and if your server is listening to port 25, 587 and 465 which are the smtp ports unless you have specified some custom port no.
# netstat -ntlp | grep sendmail
tcp 0 0.0.0.0:587 0.0.0.0:* LISTEN 1419/sendmail
tcp 0 0 0.0.0.0:465 0.0.0.0:* LISTEN 1419/sendmail
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 1419/sendmail
Many a times it happens that your network firewall might be blocking some particular port so you can check using telnet.
Solution:
Make the below changes in your sendmail.mc file
Comment out the below shown line as this will restrict your mail server to only listen to localhost loopback address and no any other address
NOTE: In sendmail "#" is not used to comment out any line. You need to use "dnl" or "dnl #"
dnl DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
or
instead of commenting the above line you can change it to
DAEMON_OPTIONS(`Port=smtp,Addr=0.0.0.0, Name=MTA')dnl
In case you are using port 587 uncomment this line
DAEMON_OPTIONS(`Port=submission, Name=MSA, M=Ea')dnl
and for **port 465** uncomment below line
DAEMON_OPTIONS(`Port=smtps, Name=TLSMTA, M=s')dnl
You can use ssl ports by adding these two lines in your sendmail.mc file
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 465')dnl Save and exit your file
# m4 sendmail.mc > sendmail.cf
# service sendmail restart
For my configuration file you can refer this post
Configuring MAIL server using sendmail
Now re-try to connect your SMTP server as in my case it is smtp.gmail.com using port no.
# telnet smtp.gmail.com 25
Trying 74.125.127.109...
Connected to smtp.gmail.com.
Escape character is '^]'.
220 mx.google.com ESMTP io2sm27316728pbc.24
So it shows connected, similarly you can check for other port no.