The purpose of this configuration is to harness the standard smtp property of ReplyTo to differ it from the source From: of the message.
The use case is this:
Alerts from the cluster come From: firstname.lastname@example.org
The ReplyTo header can be set for the specific notification, so that when a user hits reply, they can reply to the team managing that aspect of the cluster.
Say for instance you have three teams, three separate notification groups. All mails will be From:email@example.com with each notification having the property set with different team's distribution groups
e.g. ReplyTo: firstname.lastname@example.org for notification group that disseminates notifications specific to spark
ReplyTo: email@example.com for notification group that disseminates notifications specific to dfs issues/services/problems.
Upon alert arrival, these teams can hit reply, the ReplyTo field will populate from the headers so they address the appropriate team. Or, ReplyTo can be multiple users/distro groups, so when a user on the team replies that they are investigating an issue, it can ReplyTo a number of users or distro groups for notification.
Looking at the source here https://github.com/apache/ambari/blob/5460e8952729854f1c032a781c9a8de608ba4475/ambari-server/src/mai... it appears that the dispatch method would have to read the property in and call message.setReplyTo() and pass in a valid java maill address object. This would likely have to be a feature request.