Is Gmail Undo pattern enough for bulk mailing? What are the better alternatives?

We are in the early process of creating a mail client that would send out email campaigns to thousands of users (Mailchimp would be a similar product to compare) There are ideas and opinions, and one of them is adding an undo options to the mailing.

The owner of this approach convinced that this would raise the confidence level of the users, I'm defending an opposing idea which is adding an extra step that would review the mail content and recipients and ask for final approval.

While my solution adding extra friction to the flow, it also adds a higher level of confidence to the user's activities.

Now everything above is assumptions and we will test these with users but I would like to hear more about the subject, especially the personal opinions.

I use Gmail every day and personally seeing that undo functionality adding more stress in my life. It happened once or twice that I undo the email and edited a few details, but if I couldn't it wouldn't be a disaster either.

On the other hand, I think, when a user sends a bulk mail, the user needs a better understanding of what are she/she about to do.

What are your thoughts? Are there other alternatives?

PS: I read already this question about the same functionality, while I understand the assumptions here I would like to hear more.