This guide will help you to understand lists and the various options that come with it in detail:
Introduction to Lists
List Types
Confirmation Mail Options
Thank You Mail Options
Confirm Success Page
Lists are simply groups of contacts based on certain criteria. So, for a SaaS company like XYZ Corp, they may include:
Paying customers list
Trial customers list
All XYZ Corp Customers
Note that lists can be either static or dynamic. You can import a CSV file or add users manually, in which case the list behavior is static. You can also integrate with forms or use the API to continuously add new contacts, in which case the list is dynamic.
Whenever you are adding a contact to your list, you may decide to confirm the contact’s email address before starting to send emails. The reason for this is two-fold:
This ensures that your list quality remains high. You communicate only with users who are receptive to your brand..
Having malformed addresses that bounce, or sending emails to users who never subscribed to your service, can harm your company’s reputation. Users may report unsolicited emails as spam. If bounce or spam rates exceed acceptable limits, your ESP may block your account, preventing you from sending further emails for your business.
However, in some cases, confirmation may not be required. For example, if you are sending an email to your old customers and have just created a new list based on specific criteria.
Based on these scenarios, lists have two types:
Single Opt-in: This assumes that the contact has given consent to receive your emails. As a result, there is no confirmation required from the user’s end. They will start receiving emails immediately.
Double Opt-in: This assumes that the contact’s consent is required before you can start sending emails. This may be the case when you have a subscription form on your website or blog. You want to ensure that only genuine contacts are being added to your lists and that you’re communicating with people who are interested in your brand.
If you have selected Double Opt-In, you need to design the confirmation email that the subscriber will see.

The confirmation email should include {{.ConfirmationUrl}} in its content. This automatically gets replaced by a unique URL when the email is sent. When the user clicks the link, their status changes from subscribed to confirmed.
Once a contact has joined a list ( or registered at your website ) you may decide to send him / her a welcome mail. Enabling thank you options helps you do that effortlessly.

You can design the page that the user will see after confirming their subscription to a list. Note that this applies only to double opt-in lists.