It is never too late (or too early) to start an open and honest dialogue with your son about sexuality — but the earlier, the better, so try to take this opportunity.  Experimental kissing at 11 is, by the way, very normal.  It is likely to be occasional and not likely to be particularly romantic or erotic.  It’s much more likely to be a part of school-yard play than a sign of a sexually charged romance.  Middle school ideas of “girlfriend” and “boyfriend” have much less romantic meaning than they do in high school.  By ninth grade, 90 percent of kids have had some experience kissing.

It is not clear from your question whether your son showed you the e-mail or if you just happened to read it.  If your son showed you the e-mail, it may be that he is looking to you for guidance about how to handle his feelings and behavior.  Respectfully and gently, ask him how he felt about the kiss.  Did he and the girl kiss as a demonstration of affection, or did they feel pressured to do it by peers?  Preteens often worry about whether or not they are “normal,” and they have a strong desire to “fit in” with their peers. 

You can also acknowledge that new feelings about girls can be exciting and confusing at the same time.  Encourage him to act on genuine feelings, not because he needs to prove something to others.  Discuss what your values and attitudes are about romantic relationships for young people his age and discuss whatever limits you think will be helpful to him.

But if you read his e-mail without his permission, it might be better to allow him to keep his privacy around this and bring the subject up another way.  (Allowing him more privacy may be a good thing.)  You may want to look for "teachable moments" in what your son watches, reads, and hears in the media, or in what he hears from family, and from friends.  "Teachable moments" are those times when sexual issues come to the surface:  for instance, romantic or sexual story lines in TV shows, movies, or new shows — Janet Jackson’s Super Bowl performance is an example.  Teachable moments are ideal opportunities to hear your child's questions and concerns and to share your feelings and values about sexuality and sexual issues.  Ask what he thinks about the behaviors and relationships of the people involved and if your family’s values are helpful to him in understanding them.
 
Your son may seem like he is not listening, or that he does not want to hear about sexuality from you, but he is listening.  He knows that as his parent, you want what is best for him.  If you remain silent on sexuality that, too, sends a message.  Speak up and you will both be glad you did!  Check out the Parent Tips section of this website or visit http://www.plannedparenthood.org/, http://www.plannedparenthood.org/teen-talk/, http://www.mediawise.org/, http://www.advocatesforyouth.org/, or http://www.sextalk.org/ for additional resources and recommendations on how to get the conversation started.

The main message to convey to your son is that this is a normal experience and something that almost everyone his age has thought about or acted on.  Let him know that you are available to answer any questions that may come up.

It might be most effective to address sexuality in general by beginning the conversation with him about what knowledge he has about the topic.  You can start this by talking about a TV or movie scene or an advertisement.  In today's society, he is seeing a lot of messages about sexuality that are conflicting.  But by starting this conversation, you are taking a huge step in the right direction.  Being able to talk with him about this topic and truly listening to what he has to say allows him to view you as someone that he can feel comfortable talking with about sexuality.  I also think that it would be useful to answer his questions fully before addressing other concerns that you might have.  That way, you can judge what level he is on and accurately advise him based on what he's asking.

You both may feel awkward during this first conversation, but the feeling will pass as more discussions take place.  Once you have opened the door of communication around this topic, you are creating a space where you can have an open and honest ongoing dialogue.  It is apparent that you are invested in the well-being and education of your child since you are seeking advice from others to figure out the best way to open up this line of communication.  Because of this, your discussions will be more successful and your son will have a wealth of information that is vital to his health and safety.  Good luck!  After you get through the first tense moments of bringing it up, it's easy sailing from there!