Can i send an email from me.com with an email using a different domain?

I have had a dummy email (with a non me.com domain name) address created for my website and this email is redirected to my me.com account. This works fine.

But how can i send email from my dummy address? The dummy address is not gmail/hotmail or so on, it is using my website domain name. For far,i have only managed to reply using my me.com account.

Thank you, Aurélie