When incoming mail is processed, E4ward inserts a Reply-To header. This header allows masking of your REA when you reply to the email. The address in the header routes your reply back to E4ward's servers rather than directly to your recipient. E4ward replaces your REA with the alias that your recipient wrote to, and then sends it on to your recipient. This is called "address rewriting".
Normally you want this rewriting in order to hide your REA from your recipient. However in some cases you may wish to avoid this rewrite. For example, some mail services may reject the email if a header is changed or inserted, depending on the sender. For example, E4ward will automatically disables header rewrites for all mail sent by eBay or Paypal, to avoid the mail from being mistaken as phishing email by some email providers such as Gmail.
Or, you simply may not want the reply-to header added for your own reasons, eg you are masking your REA another way.
You can turn off the rewriting on a per-forward basis by checking the "No Rewrite" box when you add or change a forward. Be aware that by doing so, any replies you send to email sent to the forward, will reveal your REA (as configured in your mail client) to your recipient, unless you take other measures.