Hey @Nico80 just to clarify are you looking to send a sales email or a marketing email?
The sales email would be what you see when you either send from your inbox, or try send an email on a contact record. Marketing emails are sent from marketing > email.
You can select a marketing email to be sent from any user, you’ll find this option under the “Settings” tab in the marketing email editor. Marketing emails are sent from HubSpot’s marketing email servers.
Sales emails however are connected directly to your inbox, so it’s not possible to send an email on behalf of someone. These emails are sent directly from the inbox (IE gmail or outlook) you have connected to your HubSpot account. Allowing you to “send on behalf” of someone with these emails would fundamentally provide you access to their email account, so is not something the tool would really want functionality over. If you want to send a sales email as an email address, you need direct access to that inbox and you need to connect that account to your HubSpot account.