Currently this can be changed per project project.

Use case to change over 500 projects to a custom sender email. Changing per project using  UI from the project > project settings> Notifications> edit  is not efficient.

Current steps to change Configure Jira Cloud to send emails on behalf of your domain

 Workaround:

Possible workaround is to yse REST API to get project IDs then use REST API to update Project sender email. This can be put in a shell or python script.