You can send Transactional Email emails on behalf of clients, users, or customers, but you must add SPF and DKIM records and verify ownership of your sending domains before you can send email through your account. Transactional Email will not send any email from unverified domains or domains without valid SPF and DKIM records, including public domains like gmail.com, yahoo.com, and more.
If you need to send emails on behalf of your clients or users and cannot complete the configuration of those sending domains in your Transactional Email account, one option would be to send those messages from your own domain and to add the sender's address in the
Reply-to: header. Your emails would still be sent
From: your valid sending domain in Transactional Email, but the
Reply-to: header will instruct any recipient email client to direct replies to your user's or customer's address instead of the `From:` address. You can add this header to your message using the API or the SMTP integration.
Mail sent from unverified domains or domains without valid SPF and DKIM records will be rejected at the time of send, with the reject reason, unsigned.