Does one need to put a space between initials of the first and the second names. Which one is correct: "Sincerely, J. D." (where J. D. means John Doe) or "Sincerely, J.D."?
Yes, one puts a space there. Some people might leave out the space if they also leave out the periods, as in "Sincerely, JD". In that case, the space could be considered optional, although "Sincerely, J D" is also fine. But as soon as you use the periods, the space is a must.