I always watrn friends that my emails are cold, and to the point. I usually have to rewrite my emails twice and three times to make them nice!

If it is someone I dont know very well I take more time in writing it and being more polite otherwise I can get away with one sentance answers with people I work with on a daily basis and am freinly with. 

I think in work you can have humor but in writing it sometimes doesnt come across very well. 

You can be light with your friends and professional  at the same time, I dont see a conflict there.

If the communiication is Official though like a conformation of a proposal or an evaluation or something like that I dont think there should be any humor, friends or not.

What might help you is to tell your friends in person that business is business and pleasure is pleasure...

Good luck and let me know if you need a proof reader.

I think this is not your problem Razoon. The problem depends on the recipient. If a recipient is professional  enough, he/she can understand  that this is a business email which I believe it should not include any emotional words . People should aware the Business environment  and need to differentiate between the business and friendship relation. I saw many people who can deal with this situation  by controlling their behaviors based on the environment they involved.  you feel they have two types of charisma.