Buttondown Documentation
Buttondown's transactional emails, such as confirmation and welcome emails, can be customized using the same email CSS settings as regular emails.
You can also change the wording of the transactional emails by editing the email templates.
Buttondown's default transactional email template includes a header at the top of the email:
For a more custom look, you can add this CSS to your newsletter's email CSS settings to hide the header: