Hey Brian,
I’ve found the best way to do this is a little bit manual.
1.) Create a Contact Property called “Test Groups” with a dropdown select and A, B, C, etc for the number of groups you want to test.
2.) Export all contacts in the newletter list and create a column in the CSV with Test Groups and add A,B,C, etc evenly among the contacts.
3.) Re-import the contacts, allowing the Test Groups field to be updated.
4.) Clone and send emails to the groups separately. That will control who gets it when so they don’t randomly switch.
5.) If new subscribers come in, have a separate List for “Email Subscribers with no Test Group” and send to them outside with a different email.
I did a two month long test with a client that landed the best send time for their batch email audience at 2 p.m. on Wednesdays. No one saw that coming.