1 MLP is Minimum Learnable Product i.e. anything which helps you learn something about what product you should be building. It can be an email that you send out pitching your product or a landing page.
2. MVP is Minimum Viable Product i.e. by viability we imply usability. If it is not usable then it is not viable. So landing pages, wireframes are ruled out. Mailers might work if you start to provide a service (which will get eventually productized, automated, etc) via the mailer. So an ugly looking product would also do as long as users find it usable.
3. MSP is Minimum Saleable Product i.e. the product based on which you can start making revenue (if you so wanted)
Instead of thinking of an MVP as a single _thing_, we just made things easy and called everything we did our "MVP" until we started doing recurring sales.
I try to think of LP's as a catalyst instead of a validation. LP results make sure you have a potential userbase to reach out to when you suddenly have your MVP in your hands. It's like getting the cohorts ready for battle. Better line them up upfront then scrambling for them when the heat is on. In that light it's more a parallel path than a part of the MVP.