Following this community discussion group as have identified the same problem for our company. 

Some invoices are showing as Open (sent)

Some are showing as Open (delivered)

Some are showing as Open (undelivered)

I can have invoices sent to the same customer at the same time as part of the batch processing to send transactions, and 2 out of 4 were delivered and two out of 4 were undelivered. 

Is there any work around in the meantime anybody know ?