Tarpitting is when a server identifies a client’s abnormal email operations, and slows down or “tarpits” the process, preventing spam. For more information: https://www.techopedia.com/definition/1722/tarpitting
With tarpitting enabled, if a sender attempts to issue commands to the mail server before the banner, the connection is dropped:
[root@localhost ~]# telnet test.corp.interworx.com 25 Trying 10.1.16.999...Connected to test.corp.interworx.com. Escape character is '^]' HELO test 554 SMTP protocol violation Connection closed by foreign host.
To enable tarpitting, follow these steps:
1. At the command line, use the text editor to view /etc/tcprules.d/tcp.smtp. It will look similar to the following:
2. At the end of the line beginning with :allow, add the following: ,GREETDELAY="5",DROP_PRE_GREET="1"
3. Run ~vpopmail/bin/clearopensmtp