When an email causes an error while being processed through Flowmailer, it will be held to fix the issue and to be sent again. All held messages are found by navigating to the 'Report' menu > 'Held messages'.
In the overview, you can filter the list based on the submitted date, (part of the) email address, or status.
The list of held messages contains a submitted (date), type (Email/SMS), Source, Flow, Recipient, and Status (New/Requeued):
By clicking one of the line items, you open message details for the specific error.