O Gmail está soltando os e-mails em silêncio. Isso parece ser uma política do Gmail. Leia isto: Estou ter problemas para enviar mensagens no Gmail
If a message you've sent never arrived at its destination, you should first check your Sent Mail. Messages that you send to a mailing list, or to an email address that's automatically forwarding your mail to Gmail, will only appear in your Sent Mail.
Por que o Gmail está fazendo isso? Essa parece ser a resposta: Práticas recomendadas para encaminhar e-mails para o Gmail
Authentication
- Do not modify, remove or shuffle message headers or modify the body of the message. DKIM or DomainKeys signatures will break if the body is altered, and all authentication information will be lost.
- Common DKIM breakages include modifying the mime boundaries, anti-virus or anti-spam programs modifying the body of the message, expanding the message recipient using LDAP, and re-encoding the message.
- Several highly-phished domains have asked Gmail to enforce strict authentication checks on their mail. These domains include eBay, Paypal, and Google. If your mail server modifies the forwarded messages from these domains, these messages will be marked as phishing.
- One symptom of this problem is that a messages from these domains forwarded through your mail server to Gmail will have an Authentication-Results header that show that DKIM did not pass, while messages sent directly to Gmail will have a passing signature.