> ## Documentation Index > Fetch the complete documentation index at: https://help.maestra.io/llms.txt > Use this file to discover all available pages before exploring further. # How to find customers with send failures or undeliveries Customers may not receive a campaign if: * **Failed to send from Maestra** (e.g., no subscription, expired consent, customer in Control Groups list, etc.); * **Failed to deliver to the recipient's email server**. Both types of events can be found in **campaign engagement tab**. ​ You can also view the **error reason** and additional details there. Campaign engagements and customers can be **filtered by these reasons**. * Find all undeliveries with the reason **Mailbox full**. * Find all send failures where the message could not be composed. * Find customers who had two or more undeliveries with the reason **Recipient rejected email due to content** for the last week.