In a development and test environment it is not good to send mails to all customers. The idea to solve that is to allow a system configuration set to send all mails to a single to address.
I found a recent discussion about this topic here https://groups.google.com/forum/#!msg/idempiere/zLcE-KYrGQQ/iboBgRQD7UYJ and some very old idea about it (in german) here http://www.adempiere.metas.de/mediawiki/index.php/E-Mail_Versand_einrichten
There is already a system configuration MAIL_SEND_BCC_TO_ADDRESS to copy mails to one single address. This new issue is about not sending to the original address.
Another use case is that an administrator wants to check the outgoing mails manually. For this it would be good to have the original to address in the mail. Perhaps it works if we use the ReplyTo header for that.