We use SocketLabs to send all of our outgoing SEOM emails to customers and want to track opens, clicks, etc. To get these metrics in SocketLabs, each message either needs an individual message ID or a campaign ID. From SocketLabs documentation:
To tag your outbound messages with these identifiers, include this data in standard email headers. The following headers are supported:
X-xsMessageId - Specifies a message id. Valid characters include 0-9, A-Z, a-z and hyphen.
X-xsMailingId - Specifies a mailing/campaign id. Valid characters include 0-9, A-Z, a-z and hyphen.