If you are doing that volume of email, I expect you have an automated system sending the
email. Based on the amount of email you are generating, it appears that to some extent your are in the business of sending email. How important to your business is it that your email get delivered. If it is important, you need to get a good handle on how your email is handled. However you implement your solution you will want to ensure proper feedback so that:
- you remove dead addresses;
- you detect misconfiguration; and
- you get notifications if you get added to a blacklist.
Setting up your own MTA is not that difficult, and once configured they tend to run with little maintenance. There are standards and conventions to be followed, and failure to follow them will hurt your ability to deliver mail reliably. Unfortunately, some of the organizations in the business of sending high volume mail for others don't follow them. Using a third party service may result in your mail being classified as spam.
I posted a rant on Running an Email Server aimed mainly at the uninformed. It explains a fair bit about what is involved in running an MTA. My Email Policy provides additional resources as do other postings I have done on Email.
For this kind of mailing consider using a dedicated sub-domain and server. The tuning for high volume bulk email may be significantly different from what you want for person-to-person email. Ensure that your accept and review email sent to the
abuse addresses. If you have invalid email addresses in your list(s) you may run into queue size issues initially. You can register your domain with many of the big volume mail hosting services like Gmail, Hotmail, etc.
EDIT: Running your own MTA for a service like this does not mean you need to run your own MX. Your MX (Mail Exchange) and MTA for regular correspondence can be handled by a third party. Use a dedicated sub-domain for handling bulk and/or automated email. Manage it according to your needs.