Reputation: This value is based on how many bounces, spam complaints and unsubscribes your emails get when sending. The more bounces and spam complaints you get compared to the amount of emails you send, the lower your reputation will be.

Soft Bounces: This is usually what happens if an email address doesn't exist. It will try and resend up to 7 times.

Hard Bounces: When an email soft bounces more than 7 times it is converted to a hard bounce, and they email address will be blocked for a time. If an email address gets blocked often it is converted to a reject.

Complaint: Complaints represent emails that were marked as spam. These email addresses will be rejected, and will not receive emails for a year.

Rejects: An email address is rejected by the email service if it has hard bounced too many times or has been marked as spam.

Does the user have an email address on their account? Check the user's profile to see if their ‘Email' field is filled in.

Does the Email Definition have 'Email Sending' turned off? You can check this by finding the Email Definition from System > Settings > Communication Settings > Email Definitions.

Is the Email Definition written incorrectly? If you have changed the template in a way that means it doesn't render, it will not send.

Does the Client have 'Notify via Email' turned off? This can be verified when editing the Client's profile; there will be a checkbox for 'Notify via Email'.

Has the email bounced? The last thing to check is to navigate to Communications > Outbound Emails and searching for the email. You can see the status of the email here, whether it was bounced, sent, or opened.