What does error: “‘OpenActivities’ is not a valid child relationship name for entity Email Message”

If you are trying to read an email message and see this error, it is related to doing an upgrade to a Match My Email installed package 4.16 or higher and not activating all features. With new package you should enable Activities for Email Message object in Salesforce which allows you to create and relate a task to an email message.

• A Salesforce System Administrator should go to Salesforce Setup > Create > Objects.
• Click on label for Email Message.
• Click Edit
• Under Optional Features section, check the box for Allow Activities to enable.
• Click Save.