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Use Reply-To header when available
When this Queue setting is enabled and inbound email has a Reply-To address, the new discussion is created with that email address as its sender.
You can enable or disable this setting in
Queue > Edit.
This feature is handy in situations where you have an external form where you ask your visitor to share their email address among other things. The form is set up to send the contents as an email to some email address, and puts the visitor email address in the Reply-To header.
This is how a Wordpress "Contact Us" form works for example.