10 million mime parts?  That sounds like the kind of thing rfc 1870 was designed to address in a more general fashion (ie: the SMTP server should be enforcing maximum message size if you are worried about DOS attacks).

1 million = 3 seconds, 10 million = "over three minutes" sounds like a linear increase, so I don't see that there is anything special about "mime parts" in this scenario.

I have no objection to PRs making the parsing more efficient, though :)