This question was asked over 2 years back but it remains unanswered:
How can I send user who posts via email a confirmation that their message was successfully received and posted?
In mailing list mode you can configure an āecho modeā
Particularly by selecting āsend an email for every new postā in your email preferences.
However, there is no mechanism built in to Discourse for a āemail confirmationā when a message is posted on the forum. I recommend you make a feature request about it.