My apologies for the late reply.
Thank you for notifying us of this issue. We're currently experiencing several issues with the Twittermail service. While we're deploying a new server to move the service to a more stable environment we're keeping an eye out on any issues occurring right now. This means that your emails to Twittermail might bounce at certain times. We hope to have resolved this within the next 24 hours.
Our apologies for any inconvenience.