Azure AD B2C - Some users do not receive verification codes to their emails

Question

Good afternoon,

I have tried searching online but I could not find a definitive answer for this as most posts just end up as dead ends. We use Azure AD B2C for creating customers as members in our tenant. The majority of users work just fine where others seem to have issues with verification codes that are either delayed or non-existent but I can see they were sent through the audit logs on the user's profile. This does not seem to be a domain issue as we have other users spread across the affected domains with no issues (aol.com, gmail.com, other specific customer domains, etc). We have also had users work just fine and then the next week, they no longer receive verification codes to their email.

We have a very specific setup and policy, we only use email addresses that the customers have provided to us, we do not have the option for SMS, authenticator app, or alternative email. Filling in these fields have no effect which seems to be by design with our policy.

We have completed a Teams call with one user and verified the email was not in their junk, spam, or being blocked on their email server. It's as if there is no trace of it at all despite the log showing it was sent.

Is there a way to get a more reliable sense that the email was in fact delivered? We utilize the email of: "msonlineservicesteam@microsoftonline.com" rather than a custom email address so I am not sure if some email clients think the email itself looks like phishing and blocks it or not. We have tried workarounds in Outlook to set a rule for some of these users where if the From address is the email above, then it goes to their inbox which works for a few days and then stops again.

Unfortunately, I don't have a great deal of detail of the original configuration but I know we use OpenID. Any assistance would be greatly appreciated. Apologies, I'm a bit of an Azure rookie so I am hoping the details above give some sort of distinction of our issue.