421 Too many concurrent SMTP connections

ERROR:Message not sent. Server replied: Service not available, closing channel
421 Too many concurrent SMTP connections; please try again later.

It means that there are to many open SMTP connections either incoming or
outgoing.The default is 20, you can raise that with the smtp_accept_max
command in your configuration file.

you can check connections through command:

cat /etc/exim.conf |grep smtp_accept_max

By default ,the value is 100 ,You can raise it to 150 then run :

Service exim restart

You can get all the IPs addresses connected to/from TCP port 25 at any given time by running the following command :

netstat -plan|grep ‘:25′|grep ESTAB

That will show the IP addresses connected to/from TCP port 25 at any given time. It’ll change by the second. But if you have 250+ connections and you think there is likely tons of connections from or to single source, you’ll be able to see that easily. Then figure out why that IP address is connecting so much.

grep xxx.xxx.xxx.xxx /var/log/exim_mainlog

# ps -auxwwf | grep exim

# netstat -ano | grep “25”

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