What is an MSG-ID?
The MSG-ID is an identifier of an email that is assigned to an e-mail both in the incoming and outgoing mail. It is used to uniquely identify an email. 

Where can I find the MSG-ID?

The MSG-ID can be found in two ways:

For incoming messages, it can be found in the email header in the line "x-antispameurope-MSGID" and it can also be displayed in the table information of the Email Live Tracking. To do this, click the cogwheel on the right side of the table and check "Msg Id". After that a new column with the MSG-ID appears in the table.

For outgoing emails, the MSG-ID can also be displayed via the table information of the Email Live Tracking. The MSG-ID is not added to the email header for outgoing emails.