So, I have tried and own all the mail clients on Appsumo, and I also pay for them instantly. As you can probably guess, I send hundreds of thousands of emails in a month.

I have been using sendnow, and while it has some stuff going for it, and I do intend to keep it, I was not a fan of not sending to gmails or Outlook and having to manually email and wait a week for warmup. I have also used mystrika, and while it's a mature email client, I was not a fan of the interface look or the price per 15k uploaded contacts. So finally I have tried Success.ai, firstly the interface clearly feels like the most mature interface of all 3. Not only that it sends to gmails and outlook just fine. I am only starting my first campaign in success.ai but so far, out of the email clients I have tried on appsumo this is the one I would bet on. However, that said, it's clearly not as mature as instantly.ai, but I will overlook that, hoping they will improve and mature the codebase over time.

I would argue this is currently the best deal on AppSumo if your looking to send cold emails.