Customer Center login info notices not delivered

We use the Customer Center and have found a disconcerting percentage of emails that SHOULD be getting deliver to an email address are apparently not making it. We use the "notify" checkbox, and there is no log of an email being sent like there is with the typical email functionality. We've had customers check their spam/junk, other boxes, etc, to no avail and to try to specifically allow the senders address. The number seems to be increasing.

If there is no log that a message was sent and we're told it wasn't received, what can we do?

There have been reports of NetSuite IP addresses being blacklisted again, and that could contribute, but this was occurring prior to the most recent blacklisting. There's also the possibility the users setting up the customers with the customer center are not actually checking the "notify" box, but this issue has persisted even after explicit direction to ensure that box is checked. Should we just stop using the automated notification and build something to handle it that logs the email?