Please see: https://support.atlassian.com/jira-cloud-administration/docs/configure-jira-cloud-to-send-emails-on-behalf-of-your-domain/

Jira Service Desk next gen does not allow you to set an outbound email like the Classic JSD did, eg JSD next-gen supports e-mail delivery only from jira@<yourdomain>.atlassian.net .

You would still be able to receive tickets in the system through your custom e-mail address but customers would receive ticket information with the jira@<yourdomain>.atlassian.net signature.

The below screen does not exist in JSD next-gen