Ideas: since you are competing on the price. add a graph showing emails/payment growing with you and mail-chimp/other strong competition side by side. next to it say "lowers is,of course, better." then you have to attack the main objection(IMHO), lower delivery rate. not sure how but you need to think about the "no one ever got fired for chosing IBM" affect and counter it. you can say something reassuring about SES. or have a quote from a user or two.