Sending an email results in: host lookup did not complete

Symptoms

You may find that you are unable to send mail to any recipient. The following error will appear when finding the messages in "WHM / Mail Delivery Reports":

host lookup did not complete

You will also see this error when testing mail deliverability to the recipient via SSH:

# exim -bt user@domain.tld
user@domain.tld cannot be resolved at this time: host lookup did not complete

Description 

This occurs when the server cannot resolve the recipients' domains and is typically due to the nameservers configured in /etc/resolv.conf or "WHM / Resolver Configuration". 

Workaround

Use "WHM / Resolver Configuration" to remove the current nameservers from /etc/resolv.conf and add new nameservers that can resolve remote domains. Please use the DNS servers given by your hosting or internet service provider when possible. If unavailable, you can use public DNS servers, such as Google's public DNS servers 8.8.8.8 and 8.8.4.4. Please note that using public DNS servers may cause RBL's such as Spamhaus to not function correctly.

Optionally, you can edit /etc/resolv.conf via SSH by following these steps:

1. Connect to the server via SSH as the root user.
2. Open the /etc/resolv.conf file using any command-line text editor you prefer (example: vi, vim, nano, etc.) 

3. Comment out the current nameservers by adding a '#' in front of each nameserver and add the new nameserver entries:

   #nameserver 192.168.0.1
   #nameserver 192.168.0.2
   nameserver 8.8.8.8
   nameserver 8.8.4.4 

4. Save and close the file.

Additional Information

What nameservers should I use in my resolv.conf file?