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Email & Sending
10 min read

SMTP & Gmail setup — troubleshooting

Detailed setup for Gmail App Passwords and any SMTP provider, with fixes for the most common errors.

  1. Choose your provider preset

    Gmail: smtp.gmail.com, port 587, STARTTLS. Outlook/Office365: smtp.office365.com, port 587, STARTTLS. SendGrid: smtp.sendgrid.net, port 587, username 'apikey'. Mailgun: smtp.mailgun.org, port 587. Zoho: smtp.zoho.com, port 587.

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  2. Gmail: create an App Password (recommended over OAuth fallback)

    1) Enable 2-Step Verification on your Google account. 2) Visit myaccount.google.com/apppasswords. 3) Create a new app password named 'EmailBot'. 4) Use your full Gmail address as the username and the 16-character app password as the SMTP password.

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  3. Custom SMTP: gather credentials

    You need: host (e.g. smtp.your-isp.com), port (587 STARTTLS or 465 SSL), username (often your full email), and password. Confirm with your provider that 'submission' is enabled for your account.

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  4. Run Test connection

    Click Test & Save. We perform an EHLO + STARTTLS/SSL handshake, AUTH LOGIN, then a tiny test send to your own address. The result panel shows the exact SMTP response so you can diagnose failures.

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  5. Common error: 535 Authentication failed

    • Wrong username/password — for Gmail use the App Password, not your Google password. • Account locked due to 'less secure apps' — switch to an App Password. • 2FA not enabled on Gmail — enable it and regenerate the App Password. • Outlook: ensure SMTP AUTH is not disabled by the tenant admin.

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  6. Common error: connection timeout / refused

    • Wrong port — try 587 (STARTTLS) instead of 465, or vice versa. • Firewall/VPN blocking outbound 587/465 — try a different network. • Provider requires an allow-listed sending IP — contact the provider.

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  7. Common error: 550 'sender not allowed' / domain mismatch

    The From: address must match the authenticated mailbox (or be an allowed alias). Check the From field on the mailbox row and align it with the SMTP username.

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  8. Email lands in Spam

    1) Add SPF + DKIM at your domain DNS for the sending domain. 2) Add a DMARC record (start with p=none). 3) Warm up the mailbox — send 20-50/day for the first week. 4) Avoid spammy words, ALL CAPS, and excessive links. 5) Always include a working unsubscribe link.

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  9. Rate-limit / quota errors (429 / 421)

    Gmail caps at ~500 sends/day per account, Outlook ~300/day. Spread sends across multiple mailboxes or use a transactional provider (SendGrid/Mailgun) for higher volume.

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  10. Still stuck? Capture diagnostics

    Open the mailbox row → View test results → Copy diagnostics. Share that JSON with support — it includes the full SMTP transcript (with passwords redacted).

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