I 2nd everything mentioned below. I have talked with Customer Support as well and the help offered is not an option. The only thing they could suggest was using a different email provider (we use Microsoft Outlook). I'm sure you can understand, in the business world, changing email providers is not an easy fix as your suggestion makes it seem. When you have hundreds of invoices to send, its quite frustrating, not to mention time consuming, to go through and individually send invoices to clients to avoid the embarrassment when a client gets an odd looking email that sometimes gets mistaken for spam.  

I've read several other threads about this same issue, I don't believe we are the only 2 clients experiencing this.