When Sprwt resolves the selected groups into people it skips:

• registered customers who are suspended;
• registered customers and contacts whose Receive Email setting is off (customers can untick it under their own profile; you can toggle "Receive Email?" on a contact's edit page in Marketing > Email > Contacts);
• anyone whose email address is empty or not a valid email format;
• duplicates – each email address receives the campaign only once even if it is matched by several selected groups.
• everyone matched by the groups you put under Don't send to.

If, after all that, nobody is left, the campaign is rejected with No recipients found. Lead-form submissions are an exception in that they are not checked against a Receive Email flag, because they are raw form entries rather than contacts or registered users.