I may have accidentally marked a sender as spam. Now I don't know if I am receiving from that sender. Can I search a my account spam list and remove that sender from the list? Is there an approved sender list somewhere that overrides the spam list?
Mistakenly marking an email as spam once or even twice won't have any effect. It usually takes time and a lot of markings to get the Spam filters to react to it. Call or email the person and ask them to send you a test email to see if it gets through OK. If by some chance it goes to your Spam folder, just mark it as Not Spam and it will go to the Inbox folder.