The server has reached its limit for processing requests from your host.n451

its probably exim's ratelimiting, add your servers IP's to the whitelist in WHM > Exim Configuration

Also check the value of the 'Maximum Emails a Domain Can Send In 1 Hour' value in WHM > Tweak Settings

 

I just started having this problem with one of my servers rejecting mail from my backup MX server - despite the fact that the backup server is included in WHM >> Exim Configuration Editor >> Exim Backup Mail Hosts.

The server was just down for some scheduled maintenance and now a heap of our customers mail is stuck in the secondary MX queue - unable to be accepted by the main server.

 

I've found a temporary solution.

I've turned off the "Ratelimit: incoming SMTP connections that do not send QUIT, have recently matched an RBL, or have attacked the server." option.

The mail is being delivered now.

So adding a backup server to /etc/backupmxhosts doesn't actually whitelist it.

I'll need to find a way of turning this option back on without blocking the sMX server again.

 

Here is the option

Whitelist: Trusted Mail Hosts/IP Blocks (bypass all SMTP time checks except recipient verification)

 

I had this rate limiting issue on my backup MX server as well.

I simply added it to /etc/backupmxhosts and it worked.

However in my Exim I don't have the ability to tick the option for ...

Whitelist: Backup Mail Hosts (bypass all SMTP ratelimits).

I don't know why

Aaron

 

I found this same problem and did as below and it worked:
You can change the max smtp connection in /etc/exim.conf using below parameter :

smtp_accept_max = VALUE

The VALUE is the number you need to set according to your needs. You can experiment by increasing it a bit from its current value and see if it fulfills your requirement else you can increase it further until it works for you.