They've admitted what the problem is in the summer last year, which was actually identified by someone on the USA forums. Basically the quickbooks.com email has been flagged as a spamming email address by several of the large security providers to some of the internet providers. It's taken them a over a year not to address the problem which could be fixed by letting QBO users use their own email addresses directly from their online service which is could have been been done by now.
 
Anyone thinking that Intuit are going to fix this then you should look for another service which works.