If you want fast setup, low maintenance, and enough reliability for login codes + receipts + occasional notifications, I’d usually recommend:

Best default choice: Postmark

Why:

• Very good for transactional email specifically
• Easy to set up
• Strong deliverability reputation
• Simple API and clear docs
• Good bounce/error handling
• No unnecessary marketing-email complexity

It’s especially a good fit if you’re sending:

• login codes / OTPs
• password resets
• receipts
• account alerts
• low-volume notifications

Strong alternatives

SendGrid

Good if:

• you want a more general-purpose platform
• you may later need marketing emails too
• you don’t mind a somewhat more complex interface

Downside:

• can feel heavier than necessary for a solo side project

Mailgun

Good if:

• you like developer-oriented tooling
• you want flexibility and logs
• you may need more advanced routing/testing later

Downside:

• setup can be a bit more fiddly than Postmark

Amazon SES

Good if:

• you care most about cost
• you’re comfortable with AWS
• you don’t mind doing more setup yourself

Downside:

• cheapest, but usually not the fastest/easiest to get going with

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What actually matters

For your use case, these are the features that matter most:

1. Deliverability

This matters more than almost anything else. If your emails land in spam or get delayed, login codes become a support problem.

Look for:

• good sender reputation
• support for SPF, DKIM, and DMARC
• decent bounce/complaint handling

2. Transactional email support

You want a provider optimized for:

• one-off messages
• time-sensitive emails
• predictable sending

You do not need a full marketing suite unless you plan to grow into one.

3. Easy domain authentication

You’ll want to verify your domain and set up:

• SPF
• DKIM
• DMARC

Good providers make this straightforward.

4. Simple API / SDK

Since you’re solo and want minimal setup:

• clear docs
• easy API calls
• good Node/Python/Ruby/etc. SDKs

5. Logs and troubleshooting

When someone says “I didn’t get the code,” you need:

• message logs
• delivery/bounce status
• error reasons

6. Webhooks

Useful if you want:

• bounce tracking
• suppression handling
• delivery events
• complaint monitoring

7. Templates

Nice to have if you want to manage:

• login codes
• receipts
• notification styles

But don’t over-optimize this early.

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What you probably do not need right now

• drag-and-drop marketing editor
• complex automation workflows
• list segmentation
• A/B testing
• advanced personalization tools
• bulk campaign tools

Those are useful for marketing, not for simple app emails.

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My practical recommendation

For a solo side project:

• Use Postmark if you want the easiest “just works” experience for transactional email.
• Use Amazon SES if cost is the main concern and you’re okay with more setup.
• Use SendGrid if you want a broader platform and may later mix transactional + marketing.

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Quick rule of thumb

Choose based on this:

• Lowest hassle: Postmark
• Lowest cost: SES
• Most flexible/general: SendGrid or Mailgun

If you want, I can also give you:

1. a “best provider by stack” recommendation, or
2. a step-by-step setup checklist for your domain and app.