MsgID is a String property that represents the unique message identifier for the message. MsgID contains the value from the RFC 2822 message header 'Message-Id:', as used in the NNTP protocol. 

MsgID is normally assigned by the server that receives the message, and stored in the message when headers are processed during receipt. A value assigned to MsgID prior to transmission is likely to be ignored on the server that receives the message. 

Use UID to get the unique message identifer for POP3 and IMAP4 protocols.

UID