This issue can be caused by there being no email address to send the alert to.
Log into TaskCentre, then double click on the affected task
Double click on the SMTP step
Check if there is an email address listed here - if not, let the customer know it needs to be populated.
If the email address comes from the 'RecordSet' (i.e. the ODBC step) then further investigation will be needed to see if an email is returned in the query
Close the SMTP step window
Double click on the ODBC step
Click the Lightning Bolt icon to run the query and then click on the Results tab
Identify if there are any email addresses in the results which are blank
Note: If there are email addresses which are blank, you need to investigate the SQL query (possibly using SQL Management Studio) to see where it gets this email from and the customer will then need to populate it properly. Common causes are things like users not having emails against them or supplier/customer accounts not having emails against them.
If the email addresses are all correct, this can be caused by the mail server being unavailable. Please ensure there are no issues with your mail server and then try running the task again.