Contact Form 7 builds the envelope sender from the From header. If the address is missing, malformed, or uses a domain that the server does not accept, the mail function reports an invalid Return‑Path. The error stops the message from being sent and shows a red warning in the form.
The Return‑Path must be a syntactically correct email address. Characters outside the allowed set, an empty string, or a missing @ symbol break the header. When the address does not match the site’s domain, many mail servers reject it based on SPF or DKIM policies.
Plugins that override the mail function, such as WP Mail SMTP, may inject their own Return‑Path. If the plugin supplies an empty value or a placeholder that lacks a domain, the same error appears. Server‑level restrictions can also rewrite the envelope sender to an invalid account, causing the failure.