Sure you could do that, but I think the maintenance cost would by far outweigh any price advantage you could get. Honestly, Mandrill's pricing with 0.2$/1000 m

Sure you could do that, but I think the maintenance cost would by far outweigh any price advantage you could get. Honestly, Mandrill's pricing with 0.2$/1000 mails puts you at 200 $ for sending 1 million e-mails, which corresponds to maybe 1-2 hours of developer time. Designing, building and maintaining a system that sends mail through 4-6 different APIs will likely be much more expensive.