What is the difference between "Sender Verification" and "Sender Verification Callouts" in the Exim Configuration Manager ?

Question

What is the difference between "Sender Verification" and "Sender Verification Callouts" in the Exim Configuration Manager?

Answer

In the Exim Configuration Manager in WHM (Home »Service Configuration »Exim Configuration Manager) there are two similar options:

Sender Verification Callouts [?]
Use callouts to verify the existence of email senders. Exim will connect to the mail exchanger for a given address to verify it exists before accepting mail from it.

and

Sender Verification [?]
Verify that the domain's mail account actually exists at the origin.

I'll explain in more detail the difference between them below.

Sender Verification

Sender verification is a feature of Exim that is documented here:

Exim Access Control Lists - Address Verification

This process takes the address and processes it through the configured routers. Routers that have no_verify are skipped during this process. If the address is accepted by a router that is configured with verify_fail, verification fails for the address.

The outcome of address verification can differ based upon your server's specific router configuration and the specific address being verified. Although, probably the most common concern for the outcome of address verification with a default cPanel Exim configuration would be DNS resolution. If DNS resolution for the domain fails, address verification will fail as well. This specific portion of the configuration is responsible for the message provided when verification fails due to DNS resolution:

/etc/exim.conf

#
# Trap Failures to Remote Domain
#

fail_remote_domains:
driver = redirect
domains = ! +local_domains : ! localhost : ! localhost.localdomain
allow_fail
data = ":fail: The mail server could not deliver mail to $local_part@$domain. The account or domain may not exist, they may be blacklisted, or missing the proper dns entries."

You can learn how to test Sender Verification in the following resource: 

How to test Sender Verification with Exim

Sender Verification Callouts

Sender Verification Callouts in the Exim Configuration Manager is a setting that enhances the Sender Verification option. Sender Verification must be enabled to make use of Sender Verification Callouts.

On top of the basic verification done by the Sender Verification option, Sender Verification Callouts enables Callback Verification. 

Callback Verification is the process of making an SMTP request to the MX server of the sender to see if it actually exists.

The Exim documentation mentions this feature here:

Exim Access Control Lists - Callout Verification

You can learn how to manually perform a callout verification test here:

How to perform a test Callout / Callback  Verification