Hi @simonpurdon10,
You can accomplish this using Postmark’s API. Basically, you would leverage Postmark’s API to add a new domain into your account (i.e., your client’s domain). They would then need to verify their domain and add DKIM records along with a return path.
With the Postmark API, you can even run a call to check if the domain has been verified yet. Once the domain has been verified is when you can start sending from your client’s domain. This option allows better deliverability than the alternative I’ll go into below but does require technical setup on the user-end.
The alternative option that provides less optimal deliverability rates is adding your client’s email as a sender signature via the API. When that request is put through, Postmark will send your client an email to verify their email. Once your client verifies their email with Postmark, you can start sending, but DKIM won’t be setup, so the email won’t exactly be authenticated hence the poorer deliverability rates. However, this is less technical for your clients.