There are three different things, 1. MLP, 2. MVP, 3. MSP which kind of come in order.

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)