I'm trying to send emails but keep getting this error in my Activity or from the API: Sender Signature not defined for From address.

This error means that you are trying to send with a From address that has not been set up and confirmed as a Sender Signature in your account. In order to send using Postmark you have to confirm each address you want to use in the From field. We do this to ensure that you own the mailbox you are sending from. This prevents spoofing.

To set up a new Sender Signature go to the Sender Signatures tab in your account and click the button to Add a New Signature. Once the signature has been confirmed, you can then begin using that address to send from immediately.

If you need to send on behalf of your customer, please follow the steps in our guide.