I'm having a similar issue.  

I have two domains that I use for work emails.  Both are hosted by the same ISP.  If I send an invoice out with an email to either of those domains CC"d I don't get the CC'd copy but I can bizarrely BCC one of those domains but not the other.  Similarly if I send an invoice to myself on one of those domains I don't get the invoice.

Im also not getting my monthly billing emails from quickbooks.  However I can get invoices from other vendors via quickbooks.  Yesterday I got spent two hours on line with support who have now escalated the problem, they were able to send me emails from their quickbooks support account.  

No idea at all whats going on.