From Resend

A near-identical API shape, plus the parts that are deliberately different.

The short version

The request bodies are close enough that most migrations are a base URL and an API key.

Field mapping

Resend	OutSend
`from`	`from`
`to`	`to`
`subject`	`subject`
`html`	`html`
`text`	`text`
`reply_to`	`replyTo` (or `reply_to`)
`headers`	`headers`
`scheduled_at`	`scheduled_at`
`tags`	— not currently supported

- https://api.resend.com/emails
+ https://app.getoutsend.com/api/v1/emails

Event mapping

Resend	OutSend
`email.delivered`	`email.delivered`
`email.bounced`	`email.bounced`
`email.opened`	`email.opened`
`email.clicked`	`email.clicked`
`email.complained`	`email.complained`
`email.delivery_delayed`	`delivery_delay` event

Checklist

Verify your domain and publish DKIM, SPF, and DMARC.
Import your suppression list.
Change the base URL and the API key. Rename reply_to if you prefer the camelCase form.
Add an Idempotency-Key header to anything you retry.
Point your webhook consumer at the new signature scheme.

Being fair about it

Resend is a genuinely good product with excellent developer experience, and it offers managed dedicated IPs — we are not going to tell you otherwise. The reasons to move are specific:

The core is open source and self-hostable. You can read how sending, suppression, and reputation scoring work, run the whole thing yourself, and leave whenever you like.
Events keep the provider’s raw payload, not only a normalised summary.
No venture capital, so there is no acquisition clock and no quarter where reliability loses to growth.
Published methodology for what we measure and how — see transparency.

If those do not matter for your use case, staying put is a reasonable decision.