The Trim Subject at option allows you to restrict the number of characters that are displayed in the mail subject text field.
Click the 'Trim Subject at' drop-down box to view all the options available. You will see many numbers displayed with the limit being 80 characters. Select the numbers of characters you wish to see in the subject field of any email you receive. You also have the option of not trimming the number of character in the subject field thereby seeing the entire string in the subject field of any email you receive.