Sharing with an external user who has changed their email

I have had an issue with trying to share a folder with somebody who changed their email from, for illustrative purposes only, mailto:email address removed for privacy reasons to mailto:email address removed for privacy reasons. because his company EW had merged with Elsewhere. I believe that the second email was an alias for the first as both his actual addresses will reach him. The issue was that when I went to share a folder with his new email it would default back to his original email every single time. Eventually our IT department deleted all trace of his original email from Sharepoint which then recognised and allowed me to share with his new email. 

My question is - how does Sharepoint know these two different emails are the same person? Is it randomly deciding there is enough similarity in the email addresses- which in my view would be very dangerous, or does it know that both domains are registered to the same organisation?