When you open the properties dialog for an already created e-mail action, you will see the Exception handling tab on it.
Here you can set what happens when no recipients can be determined.
The setting Cancel transaction (default) cancels the complete process if no e-mail addresses are available.
If, for example, a Dynamic distribution list was created with the Free text type as source for the recipients,
but the data field used as a source for the current record does not contain the appropriate data, the entire process is canceled.
Select Continue transaction to ignore the error. No e-mail message will be sent. The process continues with the subsequent action.