Email delivery failure can occur for many reasons. 

Below are a few reasons why an email fails to be delivered:

• Service Disruption - There may been a disruption to your internet connection. Check your internet connection and attempt to resend the email.
• Bounced Email - The recipient’s mailbox is full or the address is non-existent. If the email address is non-existent, you should simply receive an email from the incoming server (ie. “Mailer-Daemon” or “Postmaster” alerts) informing you that the message has been discarded.
• SMTP issues - A message can always be blocked by an anti-spam filter, thus being unable to reach the recipient’s inbox. Check your SMTP server details to ensure the set up for your mail is correct. For more information on SMTP setup, see Integrations - Email.