Lead contact is the email addresses and phone numbers that are saved in the Lead View. When the lead is created, the contacts are saved to the lead view as entered. This means, in case the lead contact was incorrect, then, your outbound will surely not reach your lead.
How will I be notified that my outbound email was not sent?
You receive a notification by email that the email address of this lead is incorrect and your outbound was not sent.
How will I be notified that my outbound text was not sent?
You can see an error message in red in the lead history.
Note: You also receive an error message after you send a text if the primary phone number is a landline number.