What kind of messages will blocked senders receive?

[magicspam]

This depends on the rule that is triggered, but MagicSpam utilizes a custom system of 'verbose' bounce messages to help clearly define the policies in place.

For example, if a sender were blocked by the rule valid_helo_domain (whereby the HELO issued must be a proper domain format) then the message they would receive back is:

Your message was rejected by this system and was not delivered.
Reason: Messages from your location are rejected, bad or missing identifier (HELO). Contact your email administrator
Protection provided by: MagicSpam 1.0.8-1.1 (http://www.magicspam.com/)
For more information, please visit the URL:
http://www.linuxmagic.com/best_practice ... omain.html
or contact your ISP or mail server operator.

The links themselves give a detailed account of the rule that was triggered and also explains the terms of SMTP best practices that defines the rule.