No, warming up and verifying emails prior to sending with Letterhead is not required.
All of our customers can start sending without having to warm up their lists. Letterhead uses warm-already IP pools. However, the following strategy is perfect for granular reputation management, since it is never a bad idea to warm up a list, but your deliverability should be good from your first send with Letterhead.
If there are a high number of dead emails, this may result in a high bounce in your first newsletter metrics, but that bounce will not affect your reputation from this first send, as most of those bounces will be from the automatic recipient validation. In this first send, it will verify that the email addresses are real, unflagged, unbounced, etc. Now, please note, it does not actually verify if the user deliberately opted in. In the latter case, you can send a re-optin email as the first send, where they can unsubscribe, if they do not wish to receive your newsletter.
At the moment, pre-verification of your audience list is not possible through the Letterhead application. This is something we are considering as a new feature; however, all of our sends automatically run through a validation list first before going out into inboxes.
Email Warm-up Strategy
If you prefer to be cautious because you suspect you have a high number of "dead" emails, you can warm up your list by ramping up our send list over time. For example, start daily with sends of 10k, then 20k the following week, then 50k, etc., until you get to your entire list.
Pending Opt-in Option
You can also begin by importing your subscribers with a 'Pending' tag.
You would the create a segment labeled, 'Pending Opt-in', where the subscribers have the Pending tag, and and have your initial send be a re-optin email to the Pending segment only, drafted in our newsletter builder, that lets them know they are subscribed to the list and that they can unsubscribe if they no longer wish to receive the email.
This initial email should go out from your new Letterhead account, so that it goes out from the new sending address.