Microsoft has implemented a preview feature to its Azure cloud B2B service as described below

Microsoft added a preview feature to its Azure Active Directory Business to Business (B2B) service that makes it easier for business partners to gain access to an organization’s network resources.  The new “one-time passcodes” preview in the Azure AD B2B service, announced on Monday, works via a series of e-mail exchanges. An invitee gets an invitation containing a “Send Code” link from an organization. After the invitee clicks on that link, he or she will get a second e-mail containing a code for gaining network access, which can be used for up to 30 minutes, according to Microsoft’s documentation.

It’s a security precaution built into the one-time passcodes feature, according to the announcement:  Each authentication session only lasts 24 hours, after which guests have to re-authenticate with a new email OTP. This means your guests have to prove they still have access to their work email inboxes and have not left the partner company every 24 hours.