Hi Sandy,
If you are looking to have the To, From, and Reply-To be dynamic by lead owner for your email, you would have to use tokens (https://community.marketo.com/MarketoArticle?id=kA050000000KyqHCAS)
At my company, we add tokens to the email copy for the email signature. However, you can use snippets and add tokens to the snippet to make the email signature dynamic from the sales rep. To do this, you would include the tokens for the lead owner first and last name, the lead owner email address, and the lead owner phone number (https://community.marketo.com/MarketoArticle?id=kA050000000LBHaCAO#add-token).