Please read this article first. Then see the steps below.
Step 1: add your Things address* in Braintoss. Go to settings and then click on “add e-mail address”. Paste your Things address here.
Step 2: if this Things address is your default address in Braintoss, it will automatically send to this Things address. If you have multiple email addresses in Braintoss you can select the email address by holding the “+” button before sending.
*To use Things email address you must first enable Things Cloud. Open Things and go to the Settings or Preferences and click on Things Cloud. There enable Things Cloud. Now you can see your Things email address.
For more information: https://culturedcode.com/things/support/articles/2908262/