Normally, we write "dir" and "du" beginning with a lower case character. But in letters and E-Mails you may also write them capitalized (as you had to write it before 1996).
In 1996 we had a spelling reform, which prohibited the use of capital letters in "du" and "dir", but later on - I think it was in 2004 or 2005 - the orthography has been revised.
Since that time you may again write "Du" and "Dir" capitalized (in letters and E-Mails).
But it's always correct to write them with small letters (except at the beginning of the phrase, of course)http://www.duden.de/rechtschreibregeln/gross-un...