A couple points here on import. One is that you generally want the csv to have the real data and the real field names. So 'auto_send_smtp' with a value '1' trumps 'Auto SMTP' with a value 'Yes'. The latter is asking for problems if a field label was renamed or someone had another language in play.
Secondly, ensure that you are including an email column. The system falls back to the settings in 'Configuration » Receipts » Email Receipt' for the email and auto send fields. If you are just passing username it is not going to pick that up.
So:
username,email,auto_send_smtp
joe@acme.com,joe@acme.com,1
When you know all the usernames are email you can use email for the username label and the import should do its thing.
email,auto_send_smtp,visitor_name
joe@acme.com,1,Joe