Eh, there's a limit where it is not reasonable to continue parsing. I worked on the mailer for a huge webmail service, and it will at a certain point stop considering your inputs and respond with SMTP 5xx. Its just about knowing the limits of your software. If you knowingly write a recursive descent MIME parser then you have to also reason about the limits of how many stack frames you are permitted, or whatever limitations your language and runtime have. It's not OK to write such a parser and then just obliviously accept all inputs.