Introduction
You may use the following procedure for testing Callback / Callout Verifications
The following resource provides more context about this:
Procedure
Login to your cPanel server via SSH as the root user
Install telnet with the following command:
yum install telnet -y
Find the MX server of the domain for the address that you are testing with the following command:
dig +short domain.tld MX
Use telnet to connect to the MX server on port 25:
telnet mx1.domain.tld 25
Issue the following commands:
HELO any.hostname.tld
MAIL FROM:<>
RCPT TO:<sender@domain.tld>
If you get the following in response it means that the callback verification succeeded and the email address does exist on the server:
250 Accepted