Double Opt-In

Double opt-in is a two-step subscription flow: when someone signs up to your audience, they receive a confirmation email and only become a fully subscribed contact after clicking the confirmation link. Contacts who never click stay in your audience with status unverified and never receive campaigns. This is the standard approach for public sign-up forms, both because it improves audience quality (you only end up with addresses that work and people who actually want to be on your list) and because it’s a GDPR-friendly way to demonstrate explicit consent.

When to Use It

Use double opt-in for any contact who comes from a source you don’t directly control — the kind of sign-up where a typo, a fake address, or a bot could insert junk into your audience.

Sign-up type	Double opt-in?
Public newsletter form on your website	Yes — anyone can submit anything
Marketing landing page lead capture	Yes — same reason
In-app subscription toggle for verified users	Usually no — you already verified the email at account creation
CSV import from a trusted CRM	No — those contacts have already been verified by the upstream system
Manual entry by your team	No

Lettr’s double opt-in flow is API-only — it’s designed to be wired into your own sign-up form. There is no built-in hosted sign-up form in Lettr that uses double opt-in.

How the Flow Works

Your form calls the API

When someone submits your sign-up form, your server calls POST /api/audience/contacts with the visitor’s email and a double_opt_in block describing the confirmation email to send.

Lettr creates an unverified contact

The contact is added to your audience with status unverified and a unique confirmation token. Unverified contacts do not receive campaigns.

Lettr sends the confirmation email

Using the template you specified, Lettr sends the contact a transactional email with a {{confirmation_url}} variable that points back to Lettr’s confirmation endpoint.

The contact clicks the link

Lettr verifies the token, switches the contact’s status to subscribed, and redirects the contact’s browser to the URL you specified (typically a “Thanks for confirming!” page on your site).

The contact starts receiving campaigns

From here on, the contact is treated like any other subscribed contact in your audience.

Setting It Up

There are three pieces you need to prepare before wiring the flow into your sign-up form.

1. A Confirmation Template

Create a template for the confirmation email. It should:

Briefly explain what the recipient signed up for
Include the confirmation link using the {{confirmation_url}} merge tag
Mention what happens if they didn’t sign up (they can ignore the email)

A minimal example:

<p>Hi! Please confirm your email to join our newsletter:</p>
<p><a href="{{confirmation_url}}">Confirm your subscription</a></p>
<p>If you didn't sign up, you can ignore this email.</p>

Take note of the template’s slug — you’ll pass it in the API call.

2. A Verified Sending Domain

The from address on the confirmation email must be from a verified sending domain. If your sending domain isn’t yet verified, the confirmation email will fail to send.

3. A Redirect URL

The page on your site where Lettr will send the contact’s browser after they click the confirmation link. A simple “Thanks for confirming, you’re now subscribed” page is plenty.

Creating an Unverified Contact

Once those three pieces are ready, your sign-up handler calls the API:

curl -X POST https://app.lettr.com/api/audience/contacts \
  -H "Authorization: Bearer lttr_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" \
  -H "Content-Type: application/json" \
  -d '{
    "email": "subscriber@example.com",
    "properties": {
      "first_name": "Alex"
    },
    "double_opt_in": {
      "from": "newsletter@yourdomain.com",
      "from_name": "Acme Newsletter",
      "subject": "Confirm your subscription",
      "template_slug": "newsletter-confirmation",
      "redirect_url": "https://yourdomain.com/newsletter/confirmed"
    }
  }'

The API responds immediately. The contact is created with status unverified, and the confirmation email is queued for delivery. If you also pass properties or list_id in the call, they’re attached to the contact right away — they don’t wait for confirmation.

Required `double_opt_in` Fields

Field	Description
`from`	Sender email address (must be on a verified sending domain)
`from_name`	Display name for the sender
`subject`	Subject line of the confirmation email
`template_slug`	Slug of the template to use
`redirect_url`	URL on your site where the contact lands after clicking the link

What Recipients See

When the contact clicks the confirmation link in their inbox:

If the token is valid and the contact is still unverified, Lettr promotes them to subscribed and redirects to your redirect_url
If the contact has already confirmed (or the token is invalid), Lettr responds with an error so the contact knows the link is no longer active. The redirect doesn’t happen in this case.

You can verify the confirmation worked by looking the contact up in your dashboard — their status should be subscribed, and their activity log will show the status transition.

Best Practices

Keep the confirmation email short

The recipient signed up two seconds ago. They just want to confirm. Don’t open with marketing copy — get the link in front of them in the first paragraph.

Set expectations on the form

Tell the user on the sign-up form itself that they’ll receive a confirmation email. People are less likely to abandon a flow they know is coming.

Send confirmation from a recognizable address

The from address should match your brand. A confirmation email from noreply@some-random-tool.com is more likely to be ignored or marked spam.

Don't expect 100% confirmation

Even good flows lose 10–30% of signups at the confirmation step. That’s normal — most of the unconfirmed addresses would have been low-value contacts anyway.

Don't email unverified contacts manually

Unverified contacts are deliberately excluded from campaigns. If you start “helping them along” by emailing them through other channels, you defeat the consent purpose of double opt-in.

Single Opt-In vs. Double Opt-In

If you don’t use the double_opt_in block on the API call, the contact is created as subscribed immediately — that’s single opt-in. The trade-offs:

	Single opt-in	Double opt-in
Conversion rate	Higher (no extra step)	Lower (10–30% drop)
Audience quality	Lower (more typos, bots, fakes)	Higher (only confirmed addresses)
Deliverability over time	Worse (more bounces and complaints)	Better
GDPR posture	Weaker (consent is harder to prove)	Stronger (explicit click is a clear consent record)

For most public sign-up forms the deliverability and compliance benefits make double opt-in worth the conversion hit.

Next Steps

Contacts

See how unverified contacts work

Templates

Build your confirmation template

Sending Domains

Verify the domain you’ll send the confirmation from

Documentation

Quickstart

Learn

Integrations

Double Opt-In

When to Use It

How the Flow Works

Setting It Up

1. A Confirmation Template

2. A Verified Sending Domain

3. A Redirect URL

Creating an Unverified Contact

Required `double_opt_in` Fields

What Recipients See

Best Practices

Single Opt-In vs. Double Opt-In

Next Steps

Contacts

Templates

Sending Domains

Documentation

Quickstart

Learn

Integrations

Documentation Index

​When to Use It

​How the Flow Works

​Setting It Up

​1. A Confirmation Template

​2. A Verified Sending Domain

​3. A Redirect URL

​Creating an Unverified Contact

​Required double_opt_in Fields

​What Recipients See

​Best Practices

​Single Opt-In vs. Double Opt-In

​Next Steps

Contacts

Templates

Sending Domains

When to Use It

How the Flow Works

Setting It Up

1. A Confirmation Template

2. A Verified Sending Domain

3. A Redirect URL

Creating an Unverified Contact

Required `double_opt_in` Fields

What Recipients See

Best Practices

Single Opt-In vs. Double Opt-In

Next Steps