Emoticon :) conversion into J

I find the alphabet 'J' at the end of all the emails sent by my son. On inquiring about it he said that he did not send anything like that! Later I remembered that in the past he used to write always a smiling emoticon :) at the end of his all snail mails. Only that smiling emoticon is converted into 'J' while the email is in the process of sending! I confirmed that with my son. The question is how and why the emoticon is converted into 'J'. I first intended to end this post with a request to the readers to respond to that with a reply. But an hour's search and re-search in Google's search results provided me with the answer! My son is sending his emails from Microsoft Outlook and I am receiving them in Gmail. The emoticon :) that he sends is converted into 'J' by Gmail as it does not recognize it! That's all! You see here --> how and why of it!
I cannot say I discovered first this conversion of emoticon. Read from the links below others' experience, expressions and advices about it!
Blog post of Technet.com
What does J mean!
Techy.dstro.com
Groups.Google.com