All posts above are correct, but no one mentioned the old dot exceptions. ie .su or .fu - As most know by now, you can't have less than for characters after the dot, well there is a list of exceptions bot makers have found over the years, like the examples above. And if you are clever, you will think of an ID no one else has yet. ie
pf.fu@yahoo.com - thanks to the ID exception (.fu) you can break the no less than 6 characters in an ID rule. There is a list on my blog(see profile for link) of most ID exceptions. Warning though, using one of these ID's will get you an excessive amount of pm's regarding how to get illy's.. how to create these ID's? Just make one