How to send multiple emails at once?

I want to send a email to my users with the following conditions:

1. every email has the same content

2. addressee would see only his email address in the to field( not other addressees )

3. Mandrill and thier wpmandrill plugin are using

4. there are 40,000 addressees

Is there a better function to call rather than using a for/foreach loop?

Thanks.

P.S.: not using MailChimp because we need to send these email once a month only