If your sandbox is properly configured, you should be able to send emails. Are you having trouble sending the emails or are you able to send them but recipients are not receiving it? You should find information in the log of the delivery.
This happened to me once. Emails were not in the respective inbox though the delivery status says 'SENT'. I have then white listed the domain and it worked.
Go to Administration > Campaign Management Non Deliverables Management > Mail rule sets > Domain Management
Add your domain (xyz.com/gmail.com) and ask Adobe support team to restart the MTA to finish the whitelisting process.
You should then be receiving the emails in the inbox.