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

[MarcelB]

I keep having a large mail queue on server1 trying to send it to server2. Both servers are mine and IP's are allowed in the firewall.

When I force email on server1 to be delivered through ConfigServer Mail Queues there is the following error:
451-The server has reached its limit for processing requests from your host.n451 Please try again later.

Even a reboot of server2 didn't solve the problem.

What could be the problem here ?

[nickp666]

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

[mikelegg]

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.

[mikelegg]

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.

[nitaish]

Here is the option

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

[aarondwyer]

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

[patriciaguedes]

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.

[cPanelTristan]

I did want to note that smtp_accept_max is the maximum number of simultaneous SMTP connections on the machine for anyone who is not familiar with this setting. The default for cPanel machines in /etc/exim.conf is 100.

The proper method to revise this value would be in WHM > Exim Configuration Editor > Advanced Editor area. In the topmost box, if you input the value and click the "Save" button:

smtp_accept_max = 200

For example, it would change the value from "smtp_accept_max = 100" to the new 200 value. Any entries put at that topmost box for fields immediately below are removed and re-added with your new configuration setting.

If you manually edit /etc/exim.conf instead under cPanel, then whenever /scripts/eximup --force is processed, the value you've changed will be removed and the default set again. Thus, you would want to use WHM > Exim Configuration Editor > Advanced Editor to change the value.

To read about smtp_accept_max directive for exim, please review the following documentation from the exim site:

Exim Specification - Main configuration