Should I use an email API or just SMTP relays?

Usually, use an email API unless you have a strong reason to operate at the SMTP level.

Email API is better if you want:

• Easier integration: simpler auth, JSON requests, SDKs.
• Better deliverability tooling: bounce handling, suppression lists, retries, reputation management.
• Tracking and analytics: opens, clicks, delivery events, webhooks.
• Templates: dynamic templating, personalization, A/B testing.
• Less ops burden: no need to manage SMTP quirks, connection issues, or queueing as much.

SMTP relay is better if you want:

• Maximum compatibility with existing mail systems or legacy apps.
• Portability/vendor neutrality: SMTP is a standard, so switching providers can be easier.
• Direct integration with software already built for SMTP only.
• More control in some specialized workflows.

Practical rule of thumb

• Transactional/product emails (password resets, receipts, alerts): email API
• Legacy systems or apps already using SMTP libraries: SMTP relay
• High-scale or deliverability-sensitive sending: usually API, unless your infrastructure is already SMTP-centric

One important note

If by “SMTP relay” you mean sending through Gmail/Office365 or a random VPS SMTP server, that’s usually not ideal for production because deliverability, rate limits, and authentication can become painful.

Recommendation

For most modern applications: pick a reputable email API provider and use SMTP only if you specifically need it.

If you want, I can give you a quick comparison table of API vs SMTP for your exact use case.