This happens when you have a default signature at the bottom of your email , and is due to the fact outlook thinks your whole message is the signature due to :

A) Deleted the first two blank lines in the email to the point where the first character in your email is the first character in your signature. or

B) Hit Delete too much or used Backspace to the point where the first character of your signature is directly following the last character of your email.

The best way round this is to not do the above or add “–” to the start of every signature!