Contact Form 7 tries to send email through WordPress mail system. When the form uses a second mail template, it calls the SMTP driver that is configured by a plugin. The driver attempts to log in to the external mail server. If the server rejects the login, the form reports an authentication error.
The rejection happens because the credentials do not match what the server expects. This includes wrong username, password, or an expired app‑specific password. It also occurs when the server requires TLS or SSL but the selected encryption does not align with the port.
Some hosts block outbound SMTP traffic. In that case the connection never reaches the mail server and the driver reports a failure. A mismatched “From” address can also trigger rejection because the server only allows verified senders.
Outdated mail plugins may use an old PHPMailer version that does not support modern authentication methods. When another plugin overrides the wp_mail filter, it can pass incorrect data to PHPMailer, leading to the same error.
All of these factors stop the email from leaving the site. The form shows a red error banner and no message arrives in the inbox.