deleted preferences and lost my email in Mail

the reason you cannot find your mail is because it was nestled inside the library folder and the preference folder.
I don't know where you got the information from about deleting things to speed up your machine, but if it was a "friend", do not take any notice of anything they say regarding your PowerBook.

What you just did was to throw away everything that was anything in your account. (user, me...is Your account.) It holds all your information, documents, pictures, iTunes and a whole bunch more.

Please tell us that you did not empty the trash yet!

If you didn't, just put everything back where you got it from, restart and cross your fingers.
If you did, you might want to look around for some undeleting software which "may" rescue you.

Let us know if you already emptied the trash.

P

I did a little research on this and, if you did not get too trigger happy, you may get some of your mail back.

Take a look in the following folder. Users/you/Library/Mail (This is not the Preferences folder)

Inside there you should see a bunch of stuff that indicate the mail accounts you had. If you see stuff there, assuming you have still got that folder, try this.

Launch Mail. Under File, choose Import Mail Boxes. Choose Mail for OS X in the next window.
Navigate to that Users/you/library/mail folder and poke around until you find the mailbox folder.
Open it and choose what you need. Follow the rest of the instructions.

It's worth a try

good luck

P