Elastic Email uses MessageID as a way to uniquely identify emails. If you have a question or concern about a specific email, Elastic Email Customer Success staff can use the MessageID to quickly look up details regarding your email.
There are a few ways you can locate an email's MessageID.
From Your Email Logs
MessageIDs are available in your Email Logs. Within the UI, you just click the "eye" icon on the email. The Email Viewer will display with details about the email, including the MessageID. The same details are available in the exported Email Logs.
From Your Email Client
The MessageID is built into the email as a "header". You can find the email's MessageID right from within your email client by viewing what is often called the "original" or "source".
Google has a help document that shows you how to find this option in most major email clients.