Microsoft announced the public preview of email one-time passcodes (OTP) for Azure AD B2B (guest user) collaboration. This will enable B2B sharing with anyone with an email account.

Email OTP enables any partner which doesn’t have (or doesn’t like to use) an existing Microsoft, Office 365 or Google social ID to seamlessly share resources and collaborate without having to create a new account.

You can read the full announcement at the article below:

https://techcommunity.microsoft.com/t5/Azure-Active-Directory-Identity/Azure-AD-makes-sharing-and-collaboration-seamless-for-any-user/ba-p/325949