Testing your email before sending to thousands of recipients is not optional — it is essential. A broken image, a missing link, an unsubscribe URL that does not work, or a layout that is broken in Outlook can all be caught in minutes of pre-send testing. This article covers every testing method available, from the basic MigoSMTP dashboard preview to professional cross-client testing tools.
Why Testing Matters — The Cost of Not Testing
- A broken unsubscribe link in a marketing email is a CAN-SPAM violation.
- A password reset email that does not render the reset button on Outlook means users cannot complete the flow.
- A mis-spelled variable like
Hi {{first_name}},appearing literally because the variable was not provided — visible to every recipient. - An email wider than the screen on mobile requires horizontal scrolling — recipients delete it immediately.
- A hero image hosted on your development server (localhost) appears as a broken image to every recipient.
Layer 1 — Dashboard Preview
The fastest first check — available directly in the MigoSMTP template editor:
- Open your template in the MigoSMTP editor.
- Click the Preview button in the toolbar.
- Toggle between Desktop (600px) and Mobile (375px) preview modes.
- In preview, enter sample values for all variables to see the personalised version.
- Verify that images load, text is readable, and layout looks correct.
Dashboard preview limitation: The dashboard preview renders your email in a web browser — not in an actual email client. It will not show Outlook-specific rendering issues, font fallback behaviour, or the effects of image blocking. Always follow up with a test email send.
Layer 2 — Test Email Send
Send an actual email to your own inbox to verify real-world rendering:
- Click Send Test Email in the template editor toolbar.
- Enter one or more recipient email addresses.
- Enter sample values for all template variables.
- Click Send Test.
- Open the test email on both desktop and mobile — ideally in multiple email clients.
What to check in your inbox:
| Element | What to Verify |
|---|---|
| Subject line | Displays correctly in the inbox list; not truncated unexpectedly; personalisation shows the correct value |
| Preview text | The preheader text shows after the subject in the inbox list; no garbled characters |
| From name and address | Displays your intended From Name and verified From Address |
| Images | All images load correctly; check with images disabled to verify alt text |
| Links | Every link in the email is clickable and goes to the correct destination |
| Unsubscribe link | Click it — verify it goes to a working unsubscribe page, not a 404 |
| Variables | No {{variable_name}} strings appear literally; all are replaced with your sample values |
| Mobile layout | Open on your phone; verify no horizontal scrolling; text is readable; buttons are tappable |
| Dark mode | Switch your device to dark mode and reopen the email; check colours are still readable |
Layer 3 — Spam Score Test
Before any marketing campaign send, test your email for spam signals:
- Go to mail-tester.com and get a unique test address.
- Send your template test email to that address.
- Return to mail-tester.com and check your score.
| Score | Interpretation | Action |
|---|---|---|
| 9–10 | Excellent — very low spam risk | Proceed to send |
| 7–8 | Good — minor issues to fix | Review the flagged items and fix where possible before sending |
| 5–6 | Fair — noticeable spam risk | Fix flagged issues before sending; re-test until score improves |
| Below 5 | Poor — high spam risk | Do not send until issues are resolved; review authentication, content, and list |
Layer 4 — Cross-Client Testing
For important campaigns, test rendering in the email clients your audience uses. The major paid tools are:
| Tool | What It Does | Pricing |
|---|---|---|
| Litmus | Screenshots in 100+ email clients; spam testing; accessibility checker; load time analysis | Paid (free trial available) |
| Email on Acid | Preview in 100+ clients; automated pre-send checklist; spam filter testing | Paid (free trial available) |
| Testi@ | Real device email previews; free tier available; focus on mobile | Free tier + paid plans |
| Can I Email | HTML/CSS email compatibility lookup — check if specific CSS properties are supported | Free |
Priority email clients to test in (by usage share):
- Gmail (web) — largest global usage
- Apple Mail (macOS + iOS) — highest among premium users
- Outlook 2019 / 365 — dominant in B2B
- Gmail (Android app)
- Outlook (iOS/Android app)
Pre-Send Final Checklist
| ✓ | Item |
|---|---|
| □ | Dashboard preview looks correct on desktop and mobile |
| □ | Test email sent and verified in inbox |
| □ | All images load (no broken image icons) |
| □ | All links are working and go to correct destinations |
| □ | Unsubscribe link tested and functional |
| □ | No literal variable placeholders visible |
| □ | Spam score ≥ 7 on mail-tester.com |
| □ | Subject line and preview text display correctly in inbox |
| □ | From name and email address are correct |
| □ | Email size is under 100 KB |