Some of our reps are experiencing an error saying they can only send 1,000 mass emails a day using MSI. Is this a limitation in Marketo or SFDC?
I believe that is in both systems.
Yes, that is the limit for Marketo but you may be able to increase the daily limit through support
I would argue that before you request the limit be raised, you ask whether they're really using the right tool for the job. If the sales team is sending more than 1000 message a day, it seems like they're really being marketers. And maybe you want to use the Request Campaign functionality for some of those use cases.