Can you tell us the last part of your email address after the @ sign?
Then can you tell us how you check your email, do you go to a website or do you use a program like MacMail?
Can you tell me the part of your email address after the @ sign, is it @mac.com or @hotmail.com or something like that?
Theoretically as you are using a .Mac account someone could go to http://www.me.com/mail/ and if they knew your login information they could read your mail.
The trouble is that as there is a webmail interface like this there is no way to stop them unless you change your email password XXXXX the only way you would know they are reading your messages is if you look at your mail and see that an email you know you did not read yet is marked as being read.
I am sorry to say but if you suspect someone of reading your email all you can do is change the password XXXXX a regular basis (I change mine every 30 days) as that is the only way to stop someone else gaining access.
Let me know if you need more info or clarification