Where is the Subject line of an Email set up from?
To see this go to Admin / Manage Email Defaults / and select the relevant button. Invoices, Statements, Quotes, Receipts are all followed by either the transaction number or Account number.
To prevent emails being marked as Spam, Enter in your Company Name and then the Invoice Number.
Packing Slips, Remittance and Purchase Orders are all blank by default and should be populated.
For Letters the Subject Line is the Title of the Letter.