I just ran into this issue after 2 days of emailing invoices normally. If there were more than one email address, it was getting rejected.  I assumed maybe the person was no longer working for the company... until I ran into the issue again. 

I just recently switched from using our comcast.net email address to using Outlook, I'm still learning about Outlook so Google helped a bit, so did this post. I wasn't using semicolons but commas with spaces in between. Removing the space helped & I am at the moment able to send batch invoices with no issue. Hoping it stays this way! LOL