Have you ever sent a file or some digital photos to someone and had it bounce back because the message was too big? Every e-mail provider sets an arbitrary limit on the maximum size of an e-mail message - usually between 10mb and 20mb. A message's size is the size of the text, plus the size of the encoded attachments (encoding attachments such as photos will slightly increase the size of the files).

Why limit message sizes?
There are numerous reasons for limiting message sizes; bounced message handling, network performance, mailbox size quotas, protecting people with slower connections from receiving 100mb e-mail messages, etc.