I've had an existing email list for years, but in recent months there are increased numbers of bounces with each e-blast. Today, there is a 16.1% bounce rate. The report says many of these emails are non-existent, which is simply not the case. The emails exist and have existed for years. I also know that many of these folks have actually received the emails even though they are on the bounce list, either because I've emailed them to verify or because they have responded to the e-blast.
Your account hasn't been setup for email self-authentication, despite having your own domain. If there's been any changes to the filtering or security algorithms of your recipients, this lack of self-auth may be having an effect on the bounce triggers.