Send a test email of a campaign message to a single address. This is the v3 equivalent of the test action of the v1 campaign_send action.

The test is subject to the account's test-send limits and to sender-domain verification. Prefix the address with + to send through the regular mail server instead of the test mail server.

Responses are plain text, not JSON: 200 carries test email sent, and every caller-fixable failure (over the test limit, unverified sending domain, disallowed recipient, invalid RSS feed) carries 400 with the reason. 500 means the send failed without an attributable reason.