Invoice email recipient unreliable?

Is it just me or do invoices sent via email not always go to the correct addresses?

For a while I thought the “to” field in “New Email” always takes the address marked “primary” in customers contacts. However, that does not seem to be so completely and utterly true and invoices go to wrong places every now and then.

That is a bit of a problem, since we still dream of subscriptions with automatically sent invoices.

What is best practice here?