Hello. My site has about 15k registered users and I want to send mass mail to all who were inactive for more than a week.
I know how to check if they were online for more than a week, but the problem is how I'm going to send 15k emails without the server to crash?
How I can do it smart without wasting too much resources for 15k emails (that some of them are fake)

I need it for one time (sent to all inactive members one time)