Outlook Express 6 (XP ) cuts my mails

When I write very longue mails with Outlook Express, the receiver gets them cut,he does not get the whol mail.For short mails I have no problems,the arrive complete. With longue mails I see also in my sent items that they are cut,not the whole of my writing is there.In the ende,where the sentences are cut,I can see some carres. Why ??
Thanks for your answer.