Best Practices Troubleshooting Common MailChannels Setup Errors By MailChannels | 3 minute read Getting started with MailChannels is usually smooth, but configuration slips can happen. Whether you are integrating via SMTP or API, a small mismatch can delay critical messages like password resets, alerts, or order confirmations. This guide covers the most common MailChannels setup errors and fast fixes so you can start sending with confidence. 1. SMTP Authentication Fails Symptoms Connection errors to smtp.mailchannels.net “Authentication failed” or “535 5.7.8” in logs Causes Wrong username or password STARTTLS or SSL not enabled Using port 25 (often blocked) Fix Verify credentials in your MailChannels dashboard Use port 587 (STARTTLS) or 465 (SSL) Confirm SMTP auth is enabled in your client How to Send Transactional Email via SMTP Relay 2. Emails Not Arriving (no errors in logs) Symptoms No bounce, no delivery App says “sent” but nothing reaches the inbox Causes SPF or DKIM misconfigured Recipient server silently dropping Spam filtering at the destination Fix Confirm DNS: ✅ SPF includes your MailChannels sending domain ✅ DKIM keys published correctly ✅ From domain matches your authenticated domain Check spam folders and message headers Use inbox testing tools to trace the path Key Components of a Transactional Email 3. API Requests Return 400 or 401 Symptoms HTTP 400 (Bad Request) or 401 (Unauthorized) Causes Missing or invalid API key Wrong Content-Type or JSON body Authorization header not set Fix Set header: Authorization: Bearer YOUR_API_KEY Set Content-Type: application/json Validate payload with a JSON linter or schema How to Integrate the MailChannels Email API (with Code Examples) 4. High Bounce or Rejection Rates Symptoms Many messages bounced or rejected Low inbox placement or throttling Causes Stale or unverified addresses Sudden volume spikes without warmup Missing or incorrect domain authentication Fix Clean lists and send only to verified recipients Follow an IP warmup plan Use double opt-in to avoid typos and traps 5. Port 25 Blocked by Hosting Provider Symptoms Cannot connect to MailChannels SMTP endpoint “Connection timed out” or network errors Causes Cloud hosts (AWS, GCP, DigitalOcean) often block outbound port 25 Fix Use port 587 with STARTTLS If SMTP is blocked entirely, switch to the MailChannels Email API 6. TLS/SSL Errors Symptoms “Cannot establish TLS connection” or handshake failures Errors when sending encrypted mail Causes Outdated libraries or cipher suites Incorrect STARTTLS command order Fix Update your client to support TLS 1.2+ For SMTP, run STARTTLS before sending credentials For API, ensure HTTPS end to end without proxy interception 7. Message Blocked for Spam-Like Content Symptoms MailChannels rejects message for content Logs indicate spam filtering events Causes Subject/body triggers spam rules No unsubscribe link for non-transactional content Promotional language in transactional flows Fix Keep the message purely transactional Avoid ALL CAPS, multiple exclamation points, or click-bait subjects Use clear, action-based language like “Verify your account” or “Your receipt is ready” What Is Transactional Email vs Marketing Email Final Thoughts Tackling SPF, DKIM, ports, TLS, and payload formatting resolves most setup issues. Once configured, MailChannels provides a reliable, secure path for transactional email. Still stuck? Our support team specializes in email delivery. Reach out via your dashboard, or start fresh with the MailChannels Email API.