This option is designed primarily for remote Network Administrators. It may be of value if you run both Web- & Mail-Servers under the same IP.
If your maillog shows entries similar to the following, then this option will be your saviour:
Feb 2 05:09:47 cecilia postfix/smtpd[25844]: connect from crawl-66-249-78-241.googlebot.com[66.249.78.241] Feb 2 05:09:47 cecilia postfix/smtpd[25844]: warning: non-SMTP command from crawl-66-249-78-241.googlebot.com[66.249.78.241]: GET /robots.txt HTTP/1.1 Feb 2 05:09:47 cecilia postfix/smtpd[25844]: disconnect from crawl-66-249-78-241.googlebot.com[66.249.78.241] Feb 2 05:09:47 cecilia postfix/smtpd[25844]: connect from crawl-66-249-78-241.googlebot.com[66.249.78.241] Feb 2 05:09:47 cecilia postfix/smtpd[25844]: warning: non-SMTP command from crawl-66-249-78-241.googlebot.com[66.249.78.241]: GET / HTTP/1.1 Feb 2 05:09:47 cecilia postfix/smtpd[25844]: disconnect from crawl-66-249-78-241.googlebot.com[66.249.78.241]
Notes:
1. The above is annoying for the Webmaster & Postmaster, but what makes it diabolical is Google’s reaction to the non-result, which is to treat the entire site as “inaccessible”, and to therefore downgrade it within the Google SERPs.
2. The iptables additions are only half of the necessary fix; it will also require 301 redirections adding within the httpd conf (or Virtualhost) for the mail-host that the GoogleBot is attempting to access.
3. More information is available here (opens in a new Tab):
Fixing BrainDead GoogleBot SMTP accesses