Currently when you attempt to send a sales email to a lead that is marked as Marketing Suspended, Email Invalid, or another email send blocking "status", nothing happens once you hit send. No warning/notification is presented to the MSI user and no activity is logged on the Marketo user, giving the appearance of a "broken sync". Since no message is present on either side to indicate what happened, MSI users are unable to confirm if a mailing went out or if there was a bug in the system.
I have confirmed this occurs with the following statuses:
Unsubscribed
Email Invalid
Email Suspended
Marketing Suspended
Possible Solution:
When MSI makes the call to send the email from Marketo, if the send cannot occur or the lead is blocked from mailing, return a response code with an error of some sort.
Alternatively, these values could be shown in the MSI view in SFDC so that MSI users can be aware of what leads are available for email contact.