421 too many concurrent smtp connections

Recently there is one particular domain complaining that they can't sent out email intermittently and often receiving error "421 too many concurrent smtp connections" . When checking in the exim_mainlog, I found that their IP address always exceeding the allowed concurrently SMTP connection limit which is 100.

My question is how can I find out which user of the domain is causing the problem?

 

ok, I had just did further checking in log file and found that there is another domain is currently being attack, there are a lot of traffic coming in trying to send spams to users that do not exist in this domain. Below are some small portion of the log records.

Is this the cause of problem mentioned in my above post? if yes, how can I stop it from congesting the SMTP connections?

I have mask the recipient domain with ----.com in below log due to privacy.

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