Verification Code not emailed from PBI Signup Page

I have a couple of users who are experiencing the issue of not receiving the verification code via email on the PBI Signup page.   I have been through all the suggestions online, like checking junk folder, security settings, adding "[email protected] " to their contacts list etc.  They receive other microsoft emails, but not the one sent from the PBI signup page.  This issue is isolated to 3 out of 100 users so need to understand the cause and how to resovle.  I have reached out to the Microsoft Chat team, who were very prompt and polite, but advised this was a PBI signup platform issue not a microsoft one so unable to help.