Unable to send mail between two servers in the same IP range

Symptoms

When sending emails with two servers where the IP range is the same or similar, a connection timed out will occur.

Server 1 connecting to Server 2:

exinext domain2.com
Transport: domain2.com [10.10.10.1/10.10.10.2] error 110: Connection timed out
first failed: 05-Nov-2021 13:54:47
last tried: 05-Nov-2021 14:42:42
next try at: 05-Nov-2021 14:57:42
past final cutoff time

Server 2 connecting to Server 1:

exinext domain1.com
Transport: domain1.com [10.10.10.2/10.10.10.1] error 113: No route to host
first failed: 01-Nov-2021 07:35:45
last tried: 05-Nov-2021 07:36:12
next try at: 05-Nov-2021 15:36:12
past final cutoff time

Description 

When sending emails between servers, a link/route must be identified during the connection to reach the receiving server. If the link or route cannot be found, it will cause a connection-related error message. This issue can occur when two servers are using similar IP ranges. Commonly it happens in cPanel servers when the IP addresses are not allocated with the correct netmask.

Workaround

Ensure the IPs on the server that match another server's range are added with the proper netmask. It is best to check with the network provider to determine which network mask should communicate on each server. In many instances, the issue can be resolved by ensuring additional IPs are using netmask "255.255.255.255" if IPs in the same range will be shared with another server.

Note: It is important to check with the network provider or a certified administrator. Modifying network settings could cause the server to go offline. cPanel stores the IPs added in WHM in the following file:

/etc/ips

After changes are applied, you can restart with the restart ipaliases command:

/scripts/restartsrv_ipaliases 

Please note that cPanel, LLC only supports the cPanel-provided software and does not provide general system administration services. You may, therefore, wish to seek assistance from a qualified system administrator for this issue.

If you do not have such an administrator, you may search for one in our listings here:

System Administration Services

Please note, however, that cPanel, LLC does not endorse or recommend any particular provider on that list, nor can we be held liable for any services performed by third-party providers, including those on the list.