If you want to look into a potential do-it-yourself solution you might consider checking out Thunderbird (v17) and an add-on called Mail Merge (v3.4).
It allows you to create a template with fields to accept names, email addresses, and then set them up to launch off a mass mailing to each address one at a time. You can also set the interval between each send-off so you can disperse it over a period of a few hours.
I have it installed on Thunderbird here but haven't tried it out yet. Someone asked me this question a week ago so that's why I'm looking into this as a possible solution.