In the early days of setting up your email marketing strategy you make some fairly critical decisions that will have a significant impact later when using Interspire Email Marketer with larger and more established lists. The larger your business grows the more you may find the need to send emails to isolated contacts or segments of contacts. Initially it can be hard to decide if you should create multiple contact lists based on potential future sending or if you should have one primary list with the correct custom fields that will make it easy to send to segments of the full list based on that data.
If you want my opinion… I suggest both.
For any given opt-in campaign I run I add contacts both to the primary list and a list dedicated to that campaign or product or sales funnel. In the future its always easy for me to see how big my total list is and to send to it in one shot but if I ever need to know the source of those contacts I will quickly figure it out when I search them and find out what other campaign lists they are in. I can also send email just to contacts from that given campaign.
Where possible I would recommend always adding your contacts to a primary list and a campaign specific list. This can be easily done using our Advanced Triggers Addon. Setup your campaign to add contacts to the campaign specific contact list and then create an advanced addon to auto-subscribe contacts to the primary list as well.