What are some good options for sending receipts and system notifications from a backend app?

If you mean email receipts, password resets, invoices, alerts, and other backend-triggered messages, I’d use a transactional email provider rather than sending directly from your server.

Good options

Provider	Best for	Main trade-off
Resend	Modern apps, excellent developer experience	Less mature operational ecosystem than AWS
Postmark	Receipts and other business-critical transactional mail	More expensive; focused on transactional email
Amazon SES	Very high volume / lowest cost	More infrastructure work
Twilio SendGrid	Transactional + marketing email	Larger/more complex platform
Mailgun	Flexible email infrastructure and APIs	Pricing/DX can be less attractive for small apps

Postmark explicitly separates transactional messages such as order confirmations and password resets from broadcast mail, and provides delivery/bounce/complaint webhooks. Postmark Amazon SES provides both an API and SMTP interface and is particularly attractive when you're already heavily invested in AWS. AWS Documentation

My default recommendation

For a typical SaaS/backend app:

Resend if you want the simplest developer experience and are starting from scratch.

Postmark if a receipt or notification absolutely needs to reach the inbox and you want a provider focused specifically on transactional delivery.

SES if you're sending hundreds of thousands/millions of emails and minimizing infrastructure cost is important. The trade-off is that you take on more of the bounce, complaint, event, and template plumbing yourself. AWS Documentation+1

Architecture I'd use

Don't have business logic directly call the email provider everywhere. Instead:

Backend
   │
   ├── Create order/payment
   │
   └── enqueue "receipt.created"
             │
             ▼
        Email worker
             │
             ▼
      Email provider API
             │
             ▼
       Customer inbox

Then consume provider webhooks for delivered / bounced / complained / failed events and record those against the message in your database. This makes retries, observability, and provider migration much easier.

One other important point: keep transactional email separate from marketing/bulk email—ideally using separate streams/subdomains—so a marketing campaign doesn't damage the reputation of password resets and receipts. Zedule

If I were choosing for a new backend today: Resend → Postmark → SES, in that order depending on how much you value simplicity versus maximum control/cost efficiency.