I am not quite sure but you can check this out. Launch Outlook Express:-
Select from the top bar Tools-->Accounts and high light your Mail account. Click on Properites and select Advance. See if there is a check againt "Breakapart Larger that ....KB.
If yes remove the check or if no check the box and select appropriate number in KB.
If this is irrevelent then I am sure somebody more knowledgeable on this forum will point you to the right direction.