But to simplify in the context of email, MIME is used to separate the different components of an email; the different replies and attached files.
Each attached file is encoded as a MIME element, and each reply to an email can add another element.
Some anti-spam filters review the count of these parts, and protect themselves from abuse by limiting the number of elements found in the email.
If you experience sending errors due to this reason, the most frequent cause is the long list of replies in the email and the solution is to remove the first replies (the oldest ones) to reduce the total number of parts.