What is the simplest way to send transactional email for a SaaS app without managing SMTP?

The simplest way is to use a transactional email API service instead of running your own SMTP server.

Good options

• Postmark — very simple, great for transactional email
• SendGrid — popular, more features, a bit heavier
• Amazon SES — cheap, but more setup
• Mailgun — good API, decent for developers
• Resend — very easy for modern apps, nice developer experience

Why this is simplest

• No SMTP server to manage
• Better deliverability tools
• Easy API-based sending
• Bounce, complaint, and suppression handling included
• Templates and tracking often built in

If you want the easiest overall

• Postmark if you want “just works” transactional email
• Resend if you want a very developer-friendly modern API
• Amazon SES if cost is the priority and you don’t mind setup

Typical setup

1. Verify your sending domain
2. Set up SPF/DKIM/DMARC
3. Use the provider’s API in your app
4. Send emails like password resets, receipts, and alerts

If you want, I can recommend the best one for your stack and budget.