electronic mail from
one unique address, to another"
An eMail forward is a unique eMail address without an eMail box. In other words, when eMail is sent to an "eMail forward" address, instead of being stored in an electronic mailbox, that eMail is re-directed to another eMail address where it is stored in the mailbox belonging thereto.
Unless the owner of that specific eMail forwarding address, the sender/recipient of an eMail, is not aware of whether whomever's eMail address has or does not have a storage mailbox connected thereto and thus, eMail forward addresses can be used for convenience to re-direct mail to a vacation eMail address or to a co-worker, without revealing the destination eMail address to be other than as stated by the eMail forward address.
For example, if you're presently comfortably familiar with sending and receiving eMail via "firstname.lastname@example.org", you may find it inconvenient to memorize a new password to pick-up the eMail sent via your new registered domain name, such as "email@example.com"; in this instance, an eMail forward can be utilized, so that all eMail sent to "firstname.lastname@example.org" is automatically forwarded to "email@example.com" and thus, there is never any obligation to access other than the eMail account you are familiar with.
Another example is that of a family whose members have previously established there own, individual eMail accounts, through such as www.HotMail.com or www.Yahoo.com; after the establishment of a family website such as "www.your-family-name.com", the personalized eMail addresses associated therewith, can be forwarded to the previously established, personal eMail accounts; in other words, eMail sent to "firstname.lastname@example.org" would be automatically forwarded to "email@example.com" or "firstname.lastname@example.org" etc..