How to setup auto reply to requesters
You can set up an auto-reply generic message, letting your customers know that their request has been accepted. There is a special plugin for that, called Auto Reply To Requester.
Configuration
Being logged in as Administrator user, find it in Settings → Plugins page, in the Old Pluginssection.
At first, click Add profile.

Specify the profile name, sender’s email address, select project and set the send reply rules. Remember to click Save & Exit.
#if ($Request.RequestTypeName == "Idea")
Your idea #${Request.ID} "${Request.Name}" is accepted.
#else Your request #${Request.ID} "${Request.Name}" is accepted.
You will be notified about any progress related to the request.
#end
Need help filling in the email template body? Check Markup Guide.
Here’s what the Send reply when the request source is section is about: an auto-reply will not be sent if the drop down value for a source is 'False'. All the requests coming from Help Desk Portal and Inbound Email Integration are of the External type.
When you are done with the profile, you can optionally use Send Test Notification button.
Do not forget to enable your profile:

Setup plugin for multiple projects
You should create multiple Auto Reply to Requester profiles to configure project-specific auto-reply emails.