This question is not about how to add but I've noticed a change and native way of adding a new email alias via Exchange online mailbox properties does not seems to be working. This is what I did to add an alias.
I logged into my Tenant via GUI. Oen Exchange Admin, and from recipient I tried to modify the email properties and added anew email as an aliases. Wait for 30 min to 60 min and when I started sending emails to that mailbox on the alias, it kept on failing.
As an alternate I went to O365 Portal, in Active Users, i selected the same mailboxes and added the alias there. After 10 min 15 min I was able to send the email to the same mailbox with a newly setup alias.
Is the native Exchange Online option is no longer valid or what?