We are having problems with email groups not accepting messages sent from external addresses.  The groups are public, i.e. set to allow emails sent from anyone.  However, only email from certain domains gets through.  Email from, e.g., gmail.com or yahoo.com isn't delivered and isn't held for moderation.  If individual email addresses or domains are added to the allowed list for the group, then messages are delivered, but this shouldn't be necessary since the groups are public to begin with.  How can this be fixed?