I have a Turkish friend online.Sometimes he calls me "My girl,my lady"(we are just friends),so I call him "my boy".
One day I learned the word EFENDIM online,then I call him so.He answered me so quickly"don't call me that,I am not ur sir".
Does EFENDIM have some special meanings ? Did I do anything wrong to use this word?
I hope that somebody can explain to me.Thanks

you are not supposed to call someone you know as "Efendim"
First of all, Efendim can have two meanings.
1. Efendim means " my master, my owner. It has a negatve meaning here
2. a)Efendim indicates politness. In Turkish when the phone rings and people pick it up, then they say " Efendim"
b)

what about calling a married male friend "selam canim" when sending him an email, is this appropriate?

"selam canim" is indication of a very close relation ; closer than a normal friendship. You are not supposed to use it for that married person. It means, by the way, " my honey, my sweety". you may also use it if the relation between you and the man is different. you get what I mean