Sending Email with domain on Virtualmin

Hello, I have a VPS with Webmin/Virtualmin . I created a virtual server for on of my domain herault-entreprises.fr .
I would like to send email with this domain and an allias that I have created.
I know that I receive email at contact@herault-entreprises.fr because I see it in Postfix but how to send email with this domain ?
I tried to use roundcube but I have a 454 smtp error.
I’m kind off lost with this part.
Thanks a lot for your help

From virtualmin, open Users. Once in there click on a users name and open it. From next window you will see
button option to “login to Usermin”

Send emails from Usermin interface until you learn how to configure a mail client app like Microsoft Outlook or Thunderbird

Thanks a lot for your help