I'm using courier-imap with postfix for my virtual mailbox. Upon reading your urgent need, something that I saw before in pop3d settings came in to my mind and I immediately got back to it. Here, if you are using courier-imap, I'm sharing this.
I edited pop3d file and provided a value for DEFDOMAIN wherein by default has no value.
I restarted the daemon and tried to log-in with only the "useranme" without "@domain" using Outlook. Yes, I was able to! And to prove it further, I modified my account settings in Outlook to put only my username, I ran "Test Account Settings", and YES, it really worked!
I used this tutorial to build my postfix server.