Hi EmkT,
I've run a test on my side, but can't reproduce the issue. After I modified the alias address and send a test email to the previous alias address, I received the Undeliverable email stating that “The email address you entered couldn't be found...”. In this situation, I'd like to double confirm the following information:
1. Whether you modified the existing email alias or added a new email alias.
2. Please check whether the previous email alias still persists in the "email address" list.
Based on my research, there is not an auto mapping feature on Office 365 can do this automatically. I do suggest you submit a feedback via the following link. Your comments and suggestions will help us improve our service.
http://mymfe.microsoft.com/Microsoft%20%20Online%20%20Services/Feedback.aspx?formID=210
Thanks,
Aaron Yuan