Invoices will not send

:s For over a year I have been emailing invoices to customers with no problems. Recently I have had to resend them several times before they are received. I cc and bcc to myself and partners email which is received by both. Today I looked on my email to see where the invoice was sent. Even though the customer email was NOT typed in as gmail, it showed a gmail extension once it came to MY inbox. How is this happening!?