Each time a new email is received Postfix calculates its size in bytes. A column in the database is then simply incremented by that size when the email is forwa

Each time a new email is received Postfix calculates its size in bytes. A column in the database is then simply incremented by that size when the email is forwarded or a reply is sent. At the start of each month your bandwidth is reset to 0. Yes the size does include images and attachments.