Hi Tiemen

Currently the reply to email applies to all tenants.

There is a new feature in Totara 14 that allows email branding to be applied at the tenant level. This includes the ability to add customised headers and footers in html at a tenant level. This isn't quite what you require but this feature could be extended to include reply to address for tenants.

Are you able to cerate a feature request for this with details on your use case 

Regards