A campaign that looks perfect in your ESP preview can still fail in production. A broken link on your primary CTA, images that don't load in corporate Outlook, a clipped message in Gmail, an unsubscribe link that 404s — any one of these erodes trust and hurts future deliverability. This checklist runs through every layer systematically so nothing slips through before you hit send.
Before you start: build this into your process
Run through this checklist on every campaign, not just the high-stakes ones. The goal is to build it into muscle memory so that checking these twelve things takes five minutes, not fifty.
1. Test in at least three email clients
Gmail (web), Outlook (desktop), and Apple Mail cover roughly 65–70% of all email opens for most B2C audiences. For B2B lists, Outlook's share is often much higher. Send a test to addresses on all three before any campaign goes out. If you have budget for Litmus or Email on Acid, use them — automated rendering previews across 90+ client combinations are worth it for high-volume sends.
2. Preview on a real mobile device
Mobile accounts for 60%+ of email opens. Check that your layout collapses to a single column, that body text is at least 14px, and that CTA buttons are at least 44px tall (the minimum comfortable tap target). Pay special attention to the header area — hero images that look balanced on desktop often appear cropped or unreadable on a 375px screen.
3. Click every single link
Click every CTA, every nav link in the header, every text link in the body, every social icon in the footer, and the logo. Paste each destination URL into a browser and confirm it loads. Dead links are unprofessional and untrackable. If you're using UTM parameters, confirm they appear in your analytics platform after clicking.
4. Check all images load correctly
Images must be hosted on HTTPS — mixed-content warnings will block HTTP images in many clients. Confirm each image loads from the exact URL in the HTML, at the right dimensions, without stretching or pixelation. High-DPI assets (2×) should have explicit width and height attributes set to the display size, not the asset size.
5. Verify every image has alt text
Outlook, many corporate environments, and users with images disabled will see your alt text before anything else. An image with alt="" or no alt attribute at all will show a blank box. Make the alt text descriptive enough that the email still communicates when images are blocked — especially on your CTA buttons.
6. Check email file size
Gmail silently clips any email over 102 KB of HTML. Check your ESP's file size indicator, or copy the raw HTML into a text editor and check the character count. If you're over the limit, look for repeated inline styles, unused CSS rules, or oversize embedded images. Trim until you're safely under 90 KB.
7. Review your subject line
Mobile inbox previews cut off subject lines at roughly 41 characters. Desktop clients give you around 60. Check that the key message of your subject line lands within the first 40 characters. Avoid spam trigger words (guaranteed, free!, urgent, act now) and excessive punctuation. Don't repeat the preheader text — the two should work together, not duplicate each other.
8. Set a meaningful preheader
The preheader is the preview text displayed after your subject line in the inbox list view. If you don't set one explicitly, email clients will pull the first readable text from your email body — often something like "View this email in a browser" or a piece of boilerplate. Set it intentionally to complement your subject line and add information, not repeat it.
9. Check the plain-text version
Every HTML email should have a multipart plain-text alternative. Most ESPs generate one automatically, but auto-generated plain text is often garbled — especially if your layout uses tables. Review the plain-text version and clean it up. A messy plain-text alternative is a deliverability signal that spam filters notice.
10. Confirm the unsubscribe link works
Click your unsubscribe link and complete the opt-out flow. It should require no more than one click — requiring a password or login to unsubscribe violates CAN-SPAM and GDPR. Confirm that the address is actually suppressed in your list after opting out. A broken unsubscribe is a legal risk, not just a UX problem.
11. Check SPF, DKIM, and DMARC
Email authentication is your single biggest deliverability lever. SPF declares which mail servers are authorised to send for your domain. DKIM adds a cryptographic signature to each email. DMARC ties both together and tells receiving servers what to do when they fail. If you haven't configured all three for your sending domain, do it before your first campaign — Gmail and Yahoo now require them for bulk senders.
Use MX Toolbox or check the raw headers of a test send to confirm all three are passing.
12. Send to a seed list
Before the full send, send to a small seed list — ideally 5–10 addresses that include at least one Gmail, one Outlook, and one Apple Mail account you control. This is your final catch-all check. It catches issues that rendering previews miss: actual image load times, real inbox placement, and how your subject and preheader appear in a live inbox context.