Can I reply to comments made to my site

Generally you would leave a comment of your own. If you mean to reply personally, then you can only do that if the commenter leaves an email address.

For some reason when a commenter leaves a email address and I try to send them a email it returns to me as undeliverable.

are you sure they are legit? Spammers use false emails that frequently lead nowhere. The comments can look pretty convincing

Thanks for the answers.

Email addresses are not verified when folk leave them, so they are not guaranteed to be real or related to the person leaving the comment.