If you are getting a partial result of email sends, like the first 20 are sent fine, but then the entire rest of the list is failing, then your email preferences settings are correct, but you probably have an invalid or empty data record in the Email field for one of your records.
If the program - as it is sending - encounters an email record that is not in the form "firstname.lastname@example.org" (often because of a missing "@" symbol, or the field is simply blank), then the program's send request fails for that record and cannot continue sending data to the rest of your list.
You will want to identify the "bad" record (most likely it is the next record in the list after the successful sends stopped, like record "21" if the first 20 sent fine) and modify or delete it. Once you have fixed your list, try to send again to the remaining records.
It is key that the entire list that you are sending to has a valid email address for every record in the list. Otherwise, the program will stop sending if it encounters a missing Email field. Use the program's Filter command in the Data menu to help filter out any records that do not have email data prior to sending your mass-email.