A recent change at our SMTP server resulted in a change in credentials for the user WHMCS authenticates for SMTP.
A full day of correspondence and automated messaging was not delivered, though no obvious error was displayed to alert us to the problem. There appears to be no easy way to re-queue the day's worth of responses for automated re-send.
These were only discovered by chance viewing:
Utilities, Logs, Activity log list...
Please create an obvious error message popup when SMTP failures occurr.