Enable Email Activity Timeline for Custom Object in Experience Cloud

1. Implement a custom Apex Email Service to capture email replies and map them to the activity timeline for the custom object. This involves creating a unique identifier for the custom object record, configuring an Org-Wide Email Address, and setting up an Apex handler to process incoming emails.

2. Configure the Apex handler to process incoming emails. The handler should parse the email subject to extract the unique identifier, query the custom object for the matching record, and create an EmailMessage or Task record linked to the custom object. It should also process any attachments and save them as Salesforce Files.

3. Consider using Einstein Activity Capture (EAC) as an alternative solution. EAC now includes features that allow syncing emails as Salesforce Activities: Match Email Records and matching them to custom objects using customizable flows to personalize how those emails are attached to records (including custom objects).

4. Engage Salesforce Professional Services or a Success Guide for further assistance with implementing EAC or custom solutions for email activity capture.