When a user bulk posts and then emails a number of invoices at the end of the day, the emails aren't sending as expected.
In order to identify which emails haven't been sent, log into your data. Once you've logged in use this custom filter to see which invoices need to be resent. It will allow you to see which invoices have been posted, but aren't emailed or paid yet.
From the custom filter, bulk select all the emails that need to be resent and preview them.
From the preview page, email the invoices. This will send the emails off to your customers as expected.