The vast majority of emails, when we trace them back, report as 'delivered'.

Now 'delivered' is a many faceted term in the email world. They may have been 'delivered' into the recipients spam folder, all it means is that the email was generated, sent and the receiving server didn't throw any errors or bounce the email outright.

Unfortunately we do not have any control over the email once it leaves our server, but there are a few tricks that can help.
If you have a recipient tell you they didn't receive the emails, have them check their spam filters and move us into the trusted list of email senders. They can also add your Cavanti email address to their contacts list. Gmail have a nice summary on how to make sure the right emails get to the right place in their system here.
If it seems to be only invoices or receipts that people are not receiving, consider changing the subject line of the receipt email to something different, we have had some email providers, particularly Yahoo, objecting to the word 'payment' in the subject line and marking them as spam.

Email can be a tricky thing to navigate as anti-spam rules change very frequently and are becoming more and more strict. We will continue to do our best to stay on the right side of that line. If you have any questions or concerns please don't hesitate to contact us.