Users may see an error stating "There's a mismatch somewhere and we can't connect your email account, but we can still get you in..." when using the Salesforce Outlook Integration.
This article explains why the Outlook Integration mismatch error occurs and how to fix it by enabling API access.
The Outlook Integration with Salesforce Inbox requires "API Enabled" on the user's profile. This is because the Salesforce Inbox backend implementation uses non-Lightning servers to fetch and process email and calendar events, and uses API access to communicate back to the Lightning server. Salesforce Inbox also previously had a mobile app, which also required API access.
If users use only the Outlook Integration (without Salesforce Inbox), API access is not required.
The base Outlook Integration supports viewing related Salesforce records, and logging an email message or calendar event to Salesforce.
Salesforce Inbox adds additional productivity features on top of that:
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