On review of the example Lead we were provided where no assignment email notification was sent earlier, I've seen that the Assignment Rules were triggered by the User "Marketo Sales Insight" when the Lead was created.
The "Marketo Sales Insight" User appears to be an integration user that only logs in via the API.
When a Lead is created via the API, the API call would need to include:
* "AssignmentRuleHeader" to trigger the Assignment Rules.
* "EmailHeader" with "triggerUserEmail" set to 'true' to generate assignment notification emails.
If AssignmentRuleHeader is specified, and EmailHeader is not specified, then the records will be assigned but no assignment notifications will be sent to the record owners.
This appears to be the behavior you are encountering.
We can verify this is the case by verifying whether assignment email notifications are correctly sent/received when the Lead Assignment is triggered in the User Interface rather than by the Marketo Sales Insight integration.
If triggering assignment by creating or editing a Lead record, and saving with the "Assign using active assignment rules" box checked, does assign the Lead and successfully send an email, then we will have confirmation that the assignment rules themselves do function as expected, and the issue is definitely that the Marketo Sales Insight API integration is not triggering user emails when triggering lead assignment rules.
Note: For an application like Marketo Sales Insight, I would expect (though I cannot confirm) that there is a setting in that application which controls whether user emails are sent. That said, Marketo Sales Insight is a third party tool and is not supported by Salesforce Support, so I can not provide direction on how to locate or change such a setting. Please consult your product documentation for the Marketo Sales Insight integration, or contact Marketo Sales Insight Support if you need assistance.
If you need assistance with testing the assignment rule notifications when the rules are triggered in the User Interface, kindly confirm that I have permission to create and assign test Lead records for testing purposes.
As this post seems to be very old. So for the benefit of other readers, below is the setup available in Marketo for this issue to Send notification through Lead Assignment in Salesforce when Lead is created in Marketo and synced.
Marketo > Admin > Integration > Salesforce > Sync Options, on Trigger Email Notification select the option "Send email notification to owner in Salesforce upon lead assignment".