Email limitations are set by the host.
They normally set limits to the number of emails that can be sent over a given period.
In power Messenger, in the email settings, you can set the Number Of emails to send at a time, how long to wait between each group. use these settings so that emails don't go over the limits imposed by the host.
5 Million emails i think will be pretty difficult to handle using a system like Power Messenger unless you break the mailing into smaller chunks at a time by creating audiences.
The difficult factor will be the time that the script needs to run.
you could break it up alphabetically to create audiences based on the "UserLastName" "Begins with" "a"
you can add multiple conditions to include "OR" "UserLastName" "Begins with" "b"
This way you can send a mailing to one audience, then send the mailing again to the other audiences still you get through the entire alphabet.
We dont offer a white labeled version of power messenger, but you can create your own, and reuse those files each time you install power messenger.