You do not state how many invoices you are sending out per time frame. But, I can't see why it would not work for a large number of invoices, as it does for me with a few invoices being emailed per week from my iPad.
Here is my tip, but it requires some planning to include the Invoice Number in the SUBJECT line of your email, and not just accept the default wording from NCH. (I find very little of the default body wording helpful - deciding on my own wording & format.)
Then simply.... just view & reconcile your SENT email mailbox. The subject line displays the Invoice Number.
NB. Of course, It is habit when making an invoice, at the time of completion, email it straight away.