Bulk Email Server or SMTP relay

We throw events in Toronto, Canada and have a mailing list of 4000 (some opt in, some not) and are looking for a CHEAP service that will allow our emails to get to the inboxes and not spam .. we have the software .. we just need a mail relay or host that allows us to put our mail thru thier services with no issues .. we will send about 3 @ 4000 emails a month for 3 months .. then our parties are over .. if you know of a solution ... send us a detailed message and we will get back to you .. we can use constant contact, aweber, etc .. but they are all opt in .. and we dont have the time to wait for everyone to confirm