Go to Settings > Email Addresses:
Make sure the sender- and reply-email address matches the domain you registered for your account (e.g. domain: leadsius.com, sender/reply e-mail: firstname.lastname@example.org)
Note: You may not use email addresses based on gmail, hotmail, yahoo and similar, as these emails are likely to be treated as spam when sending email campaigns from Leadsius.
Your sender-and reply-email address may differ from you domain, in which case it needs to be approved by Leadsius support once you’ve created the email address.