What is your email stack?

I typically just use Postmark or Sendgrid for transactional email and that's about as far as I go down the email rabbit hole. However, for this upcoming project I want to rely on emails a lot more. I'll need to be able to:

• Send transactional email (reset password, billing, notifications)
• Send time/event based emails like welcome emails, onboarding flows, profile completion updates, etc.
• Send marketing emails to subscribers (new features, discounts/promotions, etc.)
• Send cold emails to prospective clients
• Possibly even allow users to email each other (since you can sort of subscribe to each other in my application)

What email services do you use to handle these various use cases?