I believe the SNFServer also using CPU would hint at there being some spam checking component to the CPU utilization overall. Does the message you are sending have complicated data in it that might require more resources to pattern match? I'd be curious to know if you see the same CPU spikes regardless of message type/content, mailing list addresses, etc. 

the SNFServer is using far less CPU than MailServer, but yes that service is the Message Sniffer for windows which checks the emails. 

The message we are sending via the mailing list system is fairly basic. We send out promo coupons for one of our clients on a bi-weekly basis. They usually contain a header region with few links and a logo, then a body consisting of 2 promo images, and finally a footer with a link to the website, un-subscribe, and various other pieces of information like copyright and address. all the images are linked back to the server, not included in the emails, so the email itself is only about 13kb. but like i said, we do have a list of about 30,000 emails (most of which are doubt opt-in)

Thanks for your follow up on this Richard. My gut tells me we're looking at a pattern matching induced increase in resource usage but grabbing a DotTrace (timeline type) of MailService.exe while the mailing list CPU usage is occurring and getting this over to our support team might allow us to identify what specifically is generating the increases.