Chris makes a great point. Adding an additional layer to your email system will allow your servers to handle this additional load.
One other possible option (which has other downsides) is to use an alternate domain for sending your Marketo emails, and set up mail on that domain on another system that isn't on your corporate network. In other words, set up hosted email at a commercial hosting service just for email for alternatedomain.com, and then send your Marketo messages from email@example.com. This would allow the other system to handle any replies.
The downside to doing this is the credibility hit you take from using an alternate domain for your marketing. If you're going to go this route, make sure the alternate domain is clearly tied to your brand, and that your branding on your messages is really tight. You might even want to send an introductory email from the old domain one last time to let people know that they can expect to receive your messages from the new domain going forward.