Exim : 451-The server has reached its limit for processing requests from your host.n451 Please try again later.

451-The server has reached its limit for processing requests from your host.n451 Please try again later.

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

retry time not reached for any host after a long failure period

exim_tidydb -t 10m /var/spool/exim retry
exim_tidydb -t 10m /var/spool/exim wait-remote_smtp

How to create a swap file

To add a swap file:

1. Determine the size of the new swap file in megabytes and multiply by 1024 to determine the number of blocks. For example, the block size of a 64 MB swap file is 65536.
   
2. At a shell prompt as root, type the following command with count being equal to the desired block size:
   

   dd if=/dev/zero of=/swapfile bs=1024 count=65536
   

3. Setup the swap file with the command:
   

   mkswap /swapfile
   

4. To enable the swap file immediately but not automatically at boot time:
   

   swapon /swapfile
   

5. To enable it at boot time, edit /etc/fstab to include the following entry:
   

   /swapfile swap swap defaults 0 0
   

   The next time the system boots, it enables the new swap file.
   
6. After adding the new swap file and enabling it, verify it is enabled by viewing the output of the command cat /proc/swaps or free.