SMTP Send Client

SMTP & Email API Test Utility

A free, native tool for testing SMTP relay servers and email API providers. Built for email administrators, developers, and IT professionals who need to quickly verify email delivery configurations.

Windows, macOS & CLI available now

Complete Email Testing Suite

Send test emails through any SMTP server or choose from 12 integrated API providers. Inspect TLS certificates, diagnose delivery issues, and securely store credentials.

🔒 Encrypted Profile Vault

Save SMTP and API credentials securely with AES-256-GCM encryption. Master password protection keeps your credentials safe between sessions.

🔐 TLS Certificate Inspection

View full certificate chains including protocol version, cipher suite, issuer details, validity dates, and SHA-256 fingerprints.

🛠 SMTP Diagnostics

24-code error database with actionable troubleshooting hints. Port connectivity testing detects ISP blocking, firewall issues, and DNS failures.

📧 HTML Email Preview

Compose in plain text or HTML with auto-detection. Live preview with rendered and raw source views. 6 preloaded templates included.

🚀 12 API Providers

One-click sending via SendGrid, Mailgun, Amazon SES, Postmark, Brevo, and 7 more providers with built-in authentication handling.

🛡 Zero Telemetry

No analytics, no tracking, no data collection. Your credentials stay local. App Sandbox enabled for maximum security.

12 Integrated Email Providers

One-click API-based sending with all major email service providers. Authentication handled automatically.

SendGrid - Twilio's cloud email platform

Mailgun - Transactional email API by Sinch

Amazon SES - AWS across 14 regions

ElasticEmail - Simple API key auth

Postmark - Fast delivery by ActiveCampaign

Brevo - Marketing & transactional platform

Mailjet - Dual API key authentication

SparkPost - High-volume delivery by Bird

SMTP2GO - Reliable SMTP and API service

SendPulse - Multi-channel with OAuth2

SocketLabs - Email infrastructure provider

Campaign Monitor - Template-based delivery

Feature Details

SMTP & API Sending

Direct SMTP relay with STARTTLS, Implicit SSL/TLS, and unencrypted modes
12 integrated API providers for one-click sending
Configurable timeouts with retry logic and exponential backoff

Email Composition

HTML and plain text with auto-detection
Live HTML preview with rendered and raw source views
6 preloaded templates (plain text, B&W HTML, colorful marketing)
Custom headers and file attachments
Multiple recipients, CC, BCC, and Reply-To support

Diagnostics & Troubleshooting

24 SMTP error codes with actionable troubleshooting hints
Port connectivity testing for ISP blocking and firewall issues
Port/TLS mismatch warnings and API key validation
Real-time color-coded activity log

Privacy & Security

AES-256-GCM encrypted vault with PBKDF2 key derivation
No analytics, no tracking, no telemetry
App Sandbox enabled for maximum security
Zero external dependencies — built with native frameworks

Download Free

No subscriptions, no limitations, no strings attached.

Windows

Windows 10/11 (x64)

Version 1.0.7.0

macOS

macOS 14.0+ (Universal)

Version 1.08

CLI Tool

Python (Cross-platform)

Open Source

Technical Specifications

macOS Version

Version: 1.08 (build 108)
Platform: macOS 14.0 (Sonoma)+
Architecture: Universal (Apple Silicon + Intel)
Frameworks: SwiftUI, CryptoKit, Network
Security: App Sandbox, Hardened Runtime

Windows Version

Version: 1.0.7.0
Platform: Windows 10/11 (x64)
Package: MSIX installer
Frameworks: .NET / WinUI
Security: Signed package, sandboxed

Security & Encryption

Vault Encryption: AES-256-GCM with PBKDF2-HMAC-SHA256 (600,000 iterations)
Transport: TLS/SSL for SMTP, HTTPS for all API calls
Authentication: SMTP AUTH LOGIN, AWS Signature V4, OAuth2, Bearer tokens, Basic Auth

Frequently Asked Questions

Is SMTP Send Client really free?

Yes, completely free with no limitations. The Windows version and CLI tool are available now. The macOS version will also be free on the Mac App Store.

What SMTP ports are supported?

All standard ports: 25 (unencrypted), 465 (Implicit SSL/TLS), 587 (STARTTLS), and 2525 (alternative). The app runs locally and includes port connectivity testing to help you determine if your ISP is blocking certain port traffic — a common issue that can prevent email delivery.

Does the app collect any data?

No. SMTP Send Client has no analytics, no tracking, and no telemetry. Everything runs locally on your device and nothing is shared with DNSai or any cloud service.

Do I have to store my credentials?

No. SMTP Send Client is designed as a test bench — you can enter credentials for a single test session without saving anything. However, since API keys and server credentials are sensitive, we offer an optional encrypted vault feature. If you choose to save server profiles, they're stored locally on your device using AES-256-GCM encryption with a master password you set. Nothing is ever shared with DNSai or any cloud service.

Can I test HTML emails?

Yes. The app includes a live HTML preview, auto-detection between HTML and plain text, and 6 preloaded email templates for thorough testing.