Use a different domain name for Email adresse

Hello,

I've purchased a Webhosting plan, but I already had the domain I wanted to use on my OVH account. During checkout I saw that I could have a free domain for 1 year with the web hosting so I took it. The thing is the web hosting comes with 10 email adresse but I can only choose to create a email adresse with the new free domain that I got, but I want to use the older domain I already had.
Any idea on how I can change the domain associated with those 10 email adress ?

Thanks for your time !