[exim] 421 Too many concurrent SMTP connections

Hello all.

I have a setup of mail system were we have one main mail server and
relay. All mail go from main to relay and then to internel - all mail
from internet go to relay and then to mail.

Relay under heavy network load - really many what to connect it.
And I start to get
"421 Too many concurrent SMTP connections; please try again later"
in logs.

But I don`t use rate limiting. What else can make this? And how can I
change it?

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Best regards,
Proskurin Kirill