What does it mean that my account may be closed?
This means that your account has been inactive for over 30 days and is at risk of being shut down.
What happens after my account is closed?
You'll lose access to all of your data and content, including your messages, split inboxes, senders you've accepted/blocked, and calendars, etc.
How can I prevent my account from being closed?
Use it! This includes sending messages from your OnMail address. You can also let your friends know about your new OnMail email address, and/or sign up for new services using your OnMail address so that you receive messages at your new address.