421 Too many concurrent SMTP connections; please try again later.

My log looks like this every day..

Reason ? Well password guessing or spam attacks.

I have a lot of connections where the remote end tries to guess or relay with username guessing, others is where they try to single point deliver spam trying to guess executive or administrative email adresses.

I would not worry about it, but you might want to tweak : Email Protection - SMTP - Advanced  - Advanced Settings

I have recently had to enable country blocking in order to keep my spam and connection attempts down, blocking top 5 spam countries that we never do business with or have users travelling to.

So, take this as your mail filter does it job - it takes the punches and dropping the connections.

Thanks for the answer  

I had to make adjustments to it. Email can be sent out almost normal. At a time when users are high.

Thanks for the answer  

I had to make adjustments to it. Email can be sent out almost normal. At a time when users are high.

Welcome :-)

Just try to keep the connections pr host to a minimum and mainly tweak max connections ( the total number of connections the proxy accepts ).

It's a long term process to tweak your mailfilter to a level where things balance.

Nice to know a lot Your answers are very important