We are having a problem with invoice sending. Our clients are not receiving their invoices. We send hundreds per day and it is becoming a major issue.

Welcome to the club. This has been an issue since February and Intuit are just not addressing what a huge problem it is for businesses. Come and join the discussion here. Ignore the accepted solution at the start of the thread as this was done by the Intuit admin and is NOT correct. It's not a solution it's an admission that they are useless and can't seem to fix the issue. 

https://quickbooks.intuit.com/learn-support/en-uk/transactions/my-emailed-invoices-are-not-being-rec...

Hello Ghost Mail, 

Welcome to the Community page, 

So our engineers are still working on getting it resolved please have a look at this link to see if it helps resolve it for you.