How to send email as from custom smtp instead from @zoho.com on behalf of...?

When I send mass email - it works. But when I tried to send the email to separate lead/contact it always appears as *@zoho.com on behalf of *@mycompany.com In email tab I have confirmed sending emails from zoho's smtp but using my email address. Why it works for mass email and doesn't work for a separate send email function? I don't want customers to see that I am sending emails through zoho, and I don't want to use mycompany's smtp. Thank you.