Marketing Cloud Engagement | Troubleshooting Emails Not Sent from Journey Builder

Emails sent through Journey Builder may not be delivered for the following reasons.

Contacts that do not match the entry criteria never enter the journey and therefore do not receive any emails.
To verify whether a contact entered the journey, check the History tab.

Common reasons a contact does not enter the journey:

Each journey requires an email address attribute to be specified during initial configuration. If the specified attribute is blank for a contact, the system cannot identify a send target, and a hard error occurs. Contacts that encounter a hard error exit the journey and do not proceed to subsequent activities.

Examples include:

For more information, see Email Address Used in Journey Builder Email Activities.

Note: In environments that use Field-Level Encryption (FLE), a population must be created before sending. See Field-Level Encrypted Email Addresses in Journey Builder.

When a job is stopped, email send requests accumulate in the queue and are not processed.
To check the job status, go to Email Studio | Interactions | Triggered Emails and select the target email activity. The send status for each journey version is displayed. When a job is stopped, the Queued count increases while no emails are sent.

1. An AMPScript error occurred during email rendering
If AMPScript errors occur repeatedly, the job stops. If Alert Manager is configured, a notification email is sent when the job stops. The subject line reads:

The notification body contains details of the error. For common error examples, see Marketing Cloud Alert Manager Errors.
Note: The RaiseError function in AMPScript can also suppress email sending. Because the boolSkipCurrentOnly parameter defaults to false, calling RaiseError without explicitly setting this parameter stops the entire job immediately. See AMPScript RaiseError.

To update email content in a running journey, follow the steps in Update an Email in a Running Journey. If the update process is not completed correctly — for example, if you navigate away before the operation finishes — the job may stop.
To resolve a stopped job, correct the AMPScript or data issue, update the email content using the procedure above, and restart the job. If restarting is not an option, run the journey as a new version instead.

Important: Triggered email queues in Journey Builder (triggered emails / triggered sends) have a 3-day expiration period. Requests that have been queued for more than 3 days are not sent after the job restarts and are treated as errors. Requests that have been queued for more than 14 days are deleted from the queue.

For more information, see Marketing Cloud Triggered Send Not Delivered to Subscribers 

As with standard bulk sends, emails are not sent to contacts with a status of Undeliverable or Unsubscribed. To identify excluded contacts, run a Tracking Extract using the [Data Extract Activity] and review the Not Sent report. For details, see this article.
Emails are also suppressed when contacts appear on a suppression list or are excluded by an exclusion script.

Q. How do I resolve a stopped job with queued requests?
Correct the AMPScript or data issue, update the email content in the running journey, and restart the job. If restarting is not an option, run the journey as a new version instead.
Note that triggered email queues have a 3-day expiration period. Requests queued for more than 3 days are not sent after the job restarts and are treated as errors. Requests queued for more than 14 days are deleted from the queue.

See also: Marketing Cloud Triggered Send Not Delivered to Subscribers 

Q. A contact passed through the email activity on the journey canvas, and the activity summary shows "Success." Does this confirm that the email was sent?
No. "Success" indicates that the email send request was successfully accepted for the contact. The email rendering and actual send processing occur asynchronously afterward. If an error occurs during that process, the email is not sent. To confirm send results, check the _Sent data view or run a Not Sent data extract, as you would for standard Email Studio sends.