When I check this box, Virtualmin adds the reject_unauth_pipelining seting to smtpd_client_restrictions. This is incorrect.
The doc on the postfix web site says this:
"Note: reject_unauth_pipelining is not useful outside smtpd_data_restrictions when 1) the client uses ESMTP (EHLO instead of HELO) and 2) with "smtpd_delay_reject = yes" (the default). The use of reject_unauth_pipelining in the other restriction contexts is therefore not recommended."
I would suggest Virtualmin be changed to add this as noted, i..e, smtpd_data_restrictions. I suppose it can stay on the same screen. It's just a matter of the generated setting.