Lenses which are classicly referred to as "true macro" do not have their own focusing mounts even if they are used for magnifications of 1:1 or higher. The 65/2.8 MP-E is a very interesting design. If I has an EOS mount camera I would get one. The test report I read on this lens showed that it was even sharper at 5:1 than at 1:1. There are many beautiful photos of plants and insects in the gallery section of photo.net which were shot with this lens. While it is true that lenses are typically optimized for a certain magnification it is not necessary to follow this advice in every case. With my Minolta Auto Bellows III outfit I find it easier to use the 25mm f/2.5 past the recommended magnification level than to use the 12.5 f/2. If I had to photograph an average size postage stamp at 1:1 I have many choices. I could use a 50 or 55 macro lens with a 1:1 tube (if needed), a 90 or 100mm lens, the 100/4 Bellows Rokkor-X, a 120/5.6 Macro Nikkor etc. All would give me good results because I am in a situation where I can control the lighting and where the subject is flat. If I need more working distance or of my subject is not flat then I might have to use something different. Robint mentions several items which I have and sometimes use. These include the Vivitar 2X Macro Focusing Teleconverter, the Panagor Auto Macro Converter and the Vivitar 135/2.8 Close Focusing lens. The Vivitar 135 goes to 1:2 by itself and is very handy when you can't get as close to a subject as you would like.
Some macro lenses like the 55/3.5 non-compensating Micro Nikkors are said to be optimized for 1:10 magnification. I find that they still work well all the way out to infinity. Two of my shorter macro lenses, the 55/2.8 AIS Nikkor and the 50/3.5 Zuiko have floating element designs and are very good over a wide range of magnifications. Just how far out of its optimized range a lens can be pushed is something you have to experiment with to find out. I don't think it would be practical to try to use my 12.5mm or 25mm Minolta micro lenses at infinity. I also enjoy using a variety of enlarging lenses, front forward and reversed, on bellows and extension tubes. Close-up and macro photography can be a lot of fun if you have the patience for it and if you like to experiment.