Username input differences between OWA and Microsoft Outlook

Hi guys,

I have a question want to ask about ms exchange .As we know there are 2 ways for accessing mailbox : OWA and Microsoft Outlook.

If I have a account like this:

Domain:dom.local

Username: u1 

Password: P@ssword

Case 1: Client use OWA for getting email and the username they have to input is:u1@dom.local

Case 2: Client use MS outook (pop3) for getting email and the username they have to input is :u1

With the case 2nd, i don't know how can they access their mailbox successfully without suffix (@dom.local). Thank you so much and looking forward from you.