The domain name of the mailbox needs to be unbound and no new domain name is bound. You can release the domain name.
Before you release the domain name, please back up your emails in advance.
You cannot be use mail.domain name to login after the release, you can replace it with https://mail.sg.aliyun.com to login.
The email accounts cannot receive and send emails after the domain name is released. If you still need to receive emails, we recommend that you replace a valid domain name. For more information about how to replace a domain name, please check Change the Domain Name of Alibaba Mail.
How to Release the Domain Name:
1. Log on to the Alibaba Mail Console, click Manage> Basic Info> Change Domain Name. 
2. A message appears on the page: If you are not using this email service any more, you can Release Domain Name. Click Release Domain Name. 
3. Click Submit.
The system will generate a new domain name for binding. Please back up the data of your email service in a timely manner. Naming rule: The new domain name will be named as jie-bang-domain name.