Subscriber keys in Salesforce Marketing Cloud
A subscriber key is a unique identifier required for all email sends. The subscriber key comprises the subscriber's email address and member ID (MID). The MID is the subscriber's business unit number.
The subscriber key appears in the following format:
For example, if John Smith's email address is
firstname.lastname@example.org and his MID is 7209944, his subscriber key is email@example.com.
If you do not know your MID, you can locate it by using the drop-down menu next to your personal settings menu. Your business unit name and MID will display.