We are running into this too...  We are using outlook with enhanced security.  it seems if the if the cc field has more than one email address, they are separated by a comma or semicolon and a space.  If we remove the trailing space after the comma or semicolon and keep both email addresses.  we DO NOT get a network failure.

[email address removed], [email address removed]  == FAILED  
[email address removed],[email address removed]  == SUCCESS.

The best part?  - if you send it without spaces.  and then SAVE the CC field when prompted...Quickbooks APPENDS a space after the comma or semicolon, basically breaking itself.