First of all when you think about tampering with mail such as remailing, forwarding etc. you think "dodgy" or is that legal? Quite simply yes, it is legal but it all depends on what intent of service is. I will explain why people need to have a fake mail address.
First of all we need to get some terms stated. I have been in business for just over two years and there is still some confusion over topic.
Mail forwarding: is a service that allows you to have your mail sent to a "mailbox" and then they will forward it off to your location. This mail is generally sent in bulk to recipient and is very useful if you travel, or don't have a permanent address.
Mail Redirection: is when you have mail sent direct to a mailbox and then it is "remailed" to another location. This is mainly used by organisations or individuals who wish to keep their physical address a secret.
Remailing: is as simple as receiving a letter, re-stamping and addressing letter then posting it again. This is pretty much what happens when mail is forwarded or redirected.
Mail Drop: is defined as "an address or place at which a nonresident person receives mail, often of a secret nature" (2005) Dictionary.com
So why are there so many names for same services? It is pretty simple to understand why there are so many names and that is because vendors that offer these services are trying to obtain as many hits through search engines as possible, they use phrases such as "forwarding" "redirection" etc. In doing this they can confuse customer and they will not have a clear understanding of services that are available.
What service should I use then? Mail drops are generally most defined term and are understandably most common. I define Mail Drops as a mailbox that has been purchased on your behalf and all mail within mailbox is sent to your predefined recipient address. Most Vendors provide shared mail boxes, where you share a box with many users for a cheap price, or dedicated services where you actually rent whole mailbox. You should look at how manageable your account is, can you have multiple predefined recipient addresses, and can you see what mail you have and how much mail you have in box at any time.