Sender ID is used to detect spoofing. A spoofed email message is modified to appear as if it originates from a sender other than the actual sender of the message. Sender ID detects spoofed email messages by using the Sender Policy Framework (SPF) record in DNS to compare the source IP address with the domain in the sender's email address.
In PoliteMail, a Sender ID is necessary in order to enable O365 Measurement.
Ideally, Sender IDs are created as part of the configuration process by IT or your Administrator, so check with them first.
Obtaining a Sender ID
Sender IDs are generated by the Actionable Email Developer Dashboard by Microsoft. This is a task best left to an Exchange Administrator.
Instructions from Microsoft's help pages may be found here.