TFWWebservices

My customer uses gmail and they hasn't been getting billing reminders and I just found today that Gmail is blocking WHMCS billing emails due to an attachment This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: ********@gmail.com host gmail-smtp-in.l.google.com [64.233.189.27] SMTP error from remote mail server after end of data: 552-5.7.0 This message was blocked because its content presents a potential 552-5.7.0 security issue. Please visit 552-5.7.0 https://support.google.com/mail/?p=BlockedMessage to review our 552 5.7.0 message content and attachment content guidelines. w2si84994990pfg.122 - gsmtp Not sure how to resolve this issue. And it hasn't been a problem till just last month. But, I checked the page listed above and .PDF files aren't on the list. <update> I un-ticked the PDF attachment version of the invoice and now it seems to be going through. But, as I said before, PDF isn't on the banned attachment list. </update>