The mailbox is sending emails out spam and the way to stop is to scan the device with an anti-virus and change the email and cPanel passwords.
Once the below is completed, please let us know to request delisting of your email address from the SMTP blocklist.
You can try Avast or AVG
To change your cPanel password;
1. Log into 1-grid customer zone
2. Click on "Services", My Service"
3. Click on "Manage Product”
4. Click Change Password
5. Enter the new password and click save changes.
To change your Email password;
1) Log into your 1-grid customer zone
2) Click on "Services", My Service"
3) Click on "Manage Product”
4) Now click on the “Login to CPanel” button;
5) Now click on the “Email Accounts” icon under the Email Tab;
6) Now you can reset the password on this page and also view emails when you choose “Access Webmail” Interface when clicking on the “More” button next to each email account.