Holds index of the message in mailbox.

An Integer value.  

object.Index

The Index Property syntax has these parts:

This is readonly property that represents position of the message in the mailbox file (if Message object specifies mail message), or it will represent position of the part in the message (if Message object specifies message part).

This value can be changed while you work with mailbox, so don't try to store it locally. For example, if you remove first message from the mailbox, all subsequent index numbers will be decreased by one.