Mail best practices?

Without going to a dedicated list for something like this, I'm looking for a common sense approach. Sep 3 17:55:20 XXX sendmail[155]: r83Lse37000155: rejecting commands from outmail016.ash2.facebook.com [66.220.155.150] due to pre-greeting traffic Sep 3 17:55:22 XXX sendmail[156]: r83Lsg6N000156: rejecting commands from outmail015.ash2.facebook.com [66.220.155.149] due to pre-greeting traffic Isn't this sort of thing supposed to be frowned upon still? I am not trying to name & shame here, but I figured this is a pretty big/respectable email sender. Thoughts for balancing sensible network spam management with sensible best practices that affect lots of users? Thanks in advance, Deepak