feat(preview-server): add --clients option to filter email client compatibility warnings by ReemX · Pull Request #2797 · resend/react-email

ReemX · 2026-01-10T22:32:11Z

Summary

Adds a new --clients CLI option to the dev command that allows users to specify which email clients to show compatibility warnings for
Useful for teams that only target specific email clients (e.g., corporate environments using only Outlook)
Also supports the COMPATIBILITY_EMAIL_CLIENTS environment variable

Usage

# Show only Outlook compatibility warnings
email dev --clients outlook

# Show warnings for multiple clients
email dev --clients "outlook,gmail,apple-mail"

Details

If --clients is not specified, defaults to ['gmail', 'apple-mail', 'outlook', 'yahoo'] (existing behavior)
Invalid client names are filtered out and ignored
If all specified clients are invalid, falls back to defaults
Supported clients: gmail, outlook, yahoo, apple-mail, aol, thunderbird, microsoft, samsung-email, sfr, orange, protonmail, hey, mail-ru, fastmail, laposte, t-online-de, free-fr, gmx, web-de, ionos-1and1, rainloop, wp-pl

Changes

packages/react-email/src/index.ts - Added --clients CLI option
packages/react-email/src/commands/dev.ts - Pass clients option through
packages/react-email/src/utils/preview/start-dev-server.ts - Pass to env builder
packages/react-email/src/utils/preview/get-env-variables-for-preview-app.ts - Add COMPATIBILITY_EMAIL_CLIENTS env var
packages/preview-server/src/app/env.ts - Export new env var
packages/preview-server/src/utils/caniemail/email-clients.ts - NEW: Extracted types and constants from server action
packages/preview-server/src/actions/email-validation/check-compatibility.ts - Use configurable email clients

Test plan

Build passes
Manual test with --clients outlook to verify only Outlook warnings shown
Manual test with invalid client names to verify fallback behavior

Closes #2796

Summary by cubic

Adds a --clients option to the dev and build commands plus a COMPATIBILITY_EMAIL_CLIENTS env var to filter compatibility warnings to chosen email clients. Keeps default clients unchanged so teams can focus on targets without noise.

New Features
- CLI: email dev --clients outlook or email dev --clients "outlook,gmail,apple-mail"
- CLI (build): supports the same --clients flag
- ENV: COMPATIBILITY_EMAIL_CLIENTS for configuration without CLI flags
- Validation: CLI errors on invalid client names; default is gmail, apple-mail, outlook, yahoo
Refactors
- Extracted email client types and list to utils/caniemail/email-clients.ts
- Threaded clients from CLI → preview env → compatibility check
- Adopted @t3-oss/env-nextjs for typed env handling and bumped zod

^{Written for commit 1692826. Summary will update on new commits.}

vercel · 2026-01-10T22:32:16Z

@ReemX is attempting to deploy a commit to the resend Team on Vercel.

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changeset-bot · 2026-01-10T22:32:16Z

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Latest commit: 1692826

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pkg-pr-new · 2026-01-10T22:33:49Z

Open in StackBlitz

npm i https://pkg.pr.new/resend/react-email/@react-email/preview-server@2797

npm i https://pkg.pr.new/resend/react-email@2797

commit: 1692826

cubic-dev-ai

2 issues found across 9 files

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="packages/react-email/src/utils/preview/start-dev-server.ts">

<violation number="1" location="packages/react-email/src/utils/preview/start-dev-server.ts:35">
P2: Port retry drops the new --clients option: compatibilityClients isn’t passed when startDevServer recurses after EADDRINUSE, so retries ignore the user’s client filter.</violation>
</file>

<file name="packages/react-email/src/utils/preview/get-env-variables-for-preview-app.ts">

<violation number="1" location="packages/react-email/src/utils/preview/get-env-variables-for-preview-app.ts:16">
P2: Undefined compatibilityClients overwrites existing COMPATIBILITY_EMAIL_CLIENTS when merging env, breaking user-provided env var support.</violation>
</file>

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packages/react-email/src/utils/preview/start-dev-server.ts

cubic-dev-ai · 2026-01-10T22:38:58Z

packages/react-email/src/utils/preview/get-env-variables-for-preview-app.ts

    PREVIEW_SERVER_LOCATION: previewServerLocation,
    USER_PROJECT_LOCATION: cwd,
    RESEND_API_KEY: resendApiKey,
+    COMPATIBILITY_EMAIL_CLIENTS: compatibilityClients,


P2: Undefined compatibilityClients overwrites existing COMPATIBILITY_EMAIL_CLIENTS when merging env, breaking user-provided env var support.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/react-email/src/utils/preview/get-env-variables-for-preview-app.ts, line 16: <comment>Undefined compatibilityClients overwrites existing COMPATIBILITY_EMAIL_CLIENTS when merging env, breaking user-provided env var support.</comment> <file context> @@ -5,12 +5,14 @@ export const getEnvVariablesForPreviewApp = ( PREVIEW_SERVER_LOCATION: previewServerLocation, USER_PROJECT_LOCATION: cwd, RESEND_API_KEY: resendApiKey, + COMPATIBILITY_EMAIL_CLIENTS: compatibilityClients, } as const; }; </file context>

Suggested change

COMPATIBILITY_EMAIL_CLIENTS: compatibilityClients,

...(compatibilityClients !== undefined

? { COMPATIBILITY_EMAIL_CLIENTS: compatibilityClients }

: {}),

…patibility warnings Adds a new `--clients` CLI option to the `dev` command that allows users to specify which email clients to show compatibility warnings for. This is useful for teams that only target specific email clients (e.g., corporate environments that only use Outlook). Usage: email dev --clients outlook email dev --clients "outlook,gmail,apple-mail" If not specified, defaults to ['gmail', 'apple-mail', 'outlook', 'yahoo']. Also supports the COMPATIBILITY_EMAIL_CLIENTS environment variable for configuration without CLI flags. Closes resend#2796

…adding one more env

cubic-dev-ai

2 issues found across 21 files (changes from recent commits).

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="packages/preview-server/src/actions/email-validation/check-compatibility.ts">

<violation number="1" location="packages/preview-server/src/actions/email-validation/check-compatibility.ts:15">
P2: COMPATIBILITY_EMAIL_CLIENTS is no longer normalized/validated; invalid/empty values now suppress all compatibility warnings instead of falling back to defaults.</violation>
</file>

<file name="packages/preview-server/src/actions/get-email-path-from-slug.ts">

<violation number="1" location="packages/preview-server/src/actions/get-email-path-from-slug.ts:10">
P1: User-controlled slug is joined without containment checks, allowing path traversal/absolute-path override outside EMAILS_DIR_ABSOLUTE_PATH before file access.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

cubic-dev-ai · 2026-01-12T14:25:11Z

packages/preview-server/src/actions/get-email-path-from-slug.ts

 export const getEmailPathFromSlug = cache(async (slug: string) => {
  if (['.tsx', '.jsx', '.ts', '.js'].includes(path.extname(slug)))
-    return path.join(emailsDirectoryAbsolutePath, slug);
+    return path.join(env.EMAILS_DIR_ABSOLUTE_PATH, slug);


P1: User-controlled slug is joined without containment checks, allowing path traversal/absolute-path override outside EMAILS_DIR_ABSOLUTE_PATH before file access.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/preview-server/src/actions/get-email-path-from-slug.ts, line 10: <comment>User-controlled slug is joined without containment checks, allowing path traversal/absolute-path override outside EMAILS_DIR_ABSOLUTE_PATH before file access.</comment> <file context> @@ -3,13 +3,13 @@ export const getEmailPathFromSlug = cache(async (slug: string) => { if (['.tsx', '.jsx', '.ts', '.js'].includes(path.extname(slug))) - return path.join(emailsDirectoryAbsolutePath, slug); + return path.join(env.EMAILS_DIR_ABSOLUTE_PATH, slug); - const pathWithoutExtension = path.join(emailsDirectoryAbsolutePath, slug); </file context>

cubic-dev-ai · 2026-01-12T14:25:11Z

packages/preview-server/src/actions/email-validation/check-compatibility.ts

  getUsedStyleProperties,
 } from '../../utils/caniemail/ast/get-used-style-properties';
+import {
+  relevantEmailClients,


P2: COMPATIBILITY_EMAIL_CLIENTS is no longer normalized/validated; invalid/empty values now suppress all compatibility warnings instead of falling back to defaults.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/preview-server/src/actions/email-validation/check-compatibility.ts, line 15: <comment>COMPATIBILITY_EMAIL_CLIENTS is no longer normalized/validated; invalid/empty values now suppress all compatibility warnings instead of falling back to defaults.</comment> <file context> @@ -13,8 +12,7 @@ import { import { - allEmailClients, - type EmailClient, + relevantEmailClients, type SupportEntry, } from '../../utils/caniemail/email-clients'; </file context>

cubic-dev-ai

1 issue found across 7 files (changes from recent commits).

Prompt for AI agents (all issues)


Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="packages/preview-server/src/app/env.ts">

<violation number="1" location="packages/preview-server/src/app/env.ts:22">
P2: `skipValidation: true` disables all env validation and default application, allowing missing/invalid env vars to reach runtime</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

cubic-dev-ai · 2026-01-12T15:01:48Z

packages/preview-server/src/app/env.ts

+      .optional()
+      .default('false'),
+  },
+  skipValidation: true,


P2: skipValidation: true disables all env validation and default application, allowing missing/invalid env vars to reach runtime

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/preview-server/src/app/env.ts, line 22: <comment>`skipValidation: true` disables all env validation and default application, allowing missing/invalid env vars to reach runtime</comment> <file context> @@ -10,12 +10,16 @@ export const env = createEnv({ + .optional() + .default('false'), }, + skipValidation: true, experimental__runtimeEnv: { NEXT_PUBLIC_IS_BUILDING: process.env.NEXT_PUBLIC_IS_BUILDING, </file context>

Suggested change

skipValidation: true,

- skipValidation: true,

+ skipValidation: process.env.SKIP_ENV_VALIDATION === 'true',

cubic-dev-ai bot reviewed Jan 10, 2026

View reviewed changes

ReemX force-pushed the feat/configurable-email-clients branch from 82d77b5 to 866fa3f Compare January 10, 2026 22:48

gabrielmfern added 3 commits January 12, 2026 11:14

move the default clients to the CLI, change env handling since we're …

9ec2e2e

…adding one more env

treat clients list properly through build too

8ee2011

default to false for is building and is preview development env

163567f

cubic-dev-ai bot reviewed Jan 12, 2026

View reviewed changes

gabrielmfern added 3 commits January 12, 2026 11:54

skip validation for it to not error during normal build

da74f60

add missing type

12765fa

add default of empty for build to not fail

be9758c

cubic-dev-ai bot reviewed Jan 12, 2026

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fix syntaxt error

1692826

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feat(preview-server): add --clients option to filter email client compatibility warnings#2797

feat(preview-server): add --clients option to filter email client compatibility warnings#2797
ReemX wants to merge 8 commits intoresend:canaryfrom
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-    COMPATIBILITY_EMAIL_CLIENTS: compatibilityClients,
+    ...(compatibilityClients !== undefined
+      ? { COMPATIBILITY_EMAIL_CLIENTS: compatibilityClients }
+      : {}),

	skipValidation: true,
	- skipValidation: true,
	+ skipValidation: process.env.SKIP_ENV_VALIDATION === 'true',

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