Thanks! I did that, but the List emails (double opt-in verification emails and welcome emails) aren't going through the server I marked for List. I'm using 1.9.18. Here's my setup:
Customer - DS1 - List
Customer - DS2 - Campaigns
System - DS3 - Transactional Emails
System - DS4 - All
I have only one Customer (myself). The Customer is allowed to use System servers. My intention is to have all List emails go through DS1. That server is more reliable. Its low quota is still more than sufficient for only List emails. All campaigns should go through DS2, which is less expensive than DS1. DS3 is copied from DS1. It seems transactional emails belong to the System, not a Customer, which makes sense. Finally, DS4 serves as a backup and catch-all in case a quota is reached or a server can't be used due to a domain policy.
However, I'm seeing in backend/misc/delivery-servers-usage-logs that more List emails went through DS2 than DS1. I'd like to have 100% of double opt-in verification emails and welcome emails go through DS1. Any idea why the server designated for Campaigns was picked for delivering List emails?