When you are just getting your account started, it is best to warm up your sending domain to avoid black lists and spam folders. This is recommended for any account that has over 1000 email contacts.
The concept is that you will send batches of emails, with increasing number of contacts, each day until you reach your full list size.
The reason for utilizing this approach is to build the reputation of a sending domain gradually so your emails will be much more likely to avoid spam filters.
The email limits are multiplied each day by the number of days (until the 8th day) the sub-account has been active.
Starting the 8th day you will have a limit set to 450,000 emails per day.