There are basically 3 types of multipart messages:

• Multipart/Mixed: Various parts of the message (except the default first one) show up as attachments within your e-mail client. This is the format used by your client to encode your e-mails when they contain attached files. Depending on the implementation you may be allowed to have cid links between the various parts of the message.
• Multipart/Related:Various parts of the message (except the default first one) are not supposed to show up as attachments (although this is implementation dependent in most of the cases), but you are allowed to have cid links between the message's various parts.
• Multipart/Alternative: Only one of the included parts actually shows up when viewing the message. Which one exactly, is determined by certain conditions such as specific settings of the mail client. For instance, the Netscape Messenger e-mail client has an option for having your e-mails sent as both HTML and text (A Multipart/Alternative message containing two parts: one with the text version of the message and one with the HTML version). If the client that receives the e-mail is set to display received e-mail in text mode only then the plain text version part of the message will be displayed.