What the Email Deliverability Preflight Checker does
This email deliverability preflight checker reads a campaign before you send it and lists what is missing or risky: required headers, From and Reply-To domains, DKIM and Return-Path alignment with your From domain, List-Unsubscribe and one-click unsubscribe headers, a visible unsubscribe link, HTML size against Gmail clipping, the balance of images and text, link domains, tracking and the plain-text part.
Each finding names where the rule comes from: RFC 5322 for message format, RFC 8058 for one-click unsubscribe, Google's and Yahoo's published sender requirements, or an A2Z heuristic. It does not send anything and does not query DNS, so it cannot tell you whether a message will reach the inbox - it tells you what you can fix before you find out.
How to use it
- Send a test of your campaign to your own mailbox and open its raw source: in Gmail choose More, then Show original; in Apple Mail choose View, Message, Raw Source; or save the message as an .eml file.
- Paste the source into the box or drop the .eml file. If you only have your template, switch to Headers + HTML and paste the HTML, plus any headers your platform shows you.
- Press Run preflight. Findings are grouped by source, with problems first.
- Fix the problems, then read the warnings. Use the SPF, DKIM and DMARC checkers linked under the results to confirm the DNS side for your From domain.
- Copy the checklist or download it as JSON to keep with the campaign record.
Reading the results
Problems (red) break a published rule: a missing From or Date header, a duplicated single-use header, one-click unsubscribe declared without an HTTPS address or without a DKIM signature over the unsubscribe headers, or a From domain that aligns with neither DKIM nor the Return-Path domain.
Warnings (amber) are things Google or Yahoo ask for that may simply be added later by your platform (Message-ID, DKIM), or content habits that filters and readers distrust, such as image-only emails and public link shorteners.
A test copy from your own inbox is the most useful input, because it shows the headers your platform really adds. A template exported before sending often has no DKIM signature or Message-ID yet, and the checker says so rather than failing it silently.
Worked example: a newsletter that is ready, and one that is not
The first example is a Northwind newsletter from [email protected]. Its DKIM signature has d=northwind.example and signs List-Unsubscribe and List-Unsubscribe-Post; the Return-Path is [email protected], whose organisational domain is also northwind.example. List-Unsubscribe has an HTTPS address and a mailto address, and List-Unsubscribe-Post is exactly List-Unsubscribe=One-Click. The result is no problems and no warnings.
The second example comes from [email protected] but is signed with d=promo-mailer.example and has no Return-Path, so the From domain aligns with neither DKIM nor SPF - a DMARC failure for bulk mail. It also has no Date header, a fake RE: subject, an http:// unsubscribe address with the One-Click header, a DKIM h= list of only From:Subject, a link to a bare IP address (203.0.113.9), a bit.ly link, link text showing www.northwind.example that points to promo-mailer.example, and one image with 5 words of text. Each of these appears as its own finding.
Which rules are requirements and which are advice
Google's email sender guidelines ask all senders to follow RFC 5322, set up SPF or DKIM, and not impersonate Gmail From: headers. Bulk senders (Google's threshold is about 5,000 messages a day to Gmail accounts) must also have SPF, DKIM and DMARC, align the From domain with the SPF or DKIM domain, support one-click unsubscribe for marketing and subscribed messages, and include a clearly visible unsubscribe link in the body. Yahoo's sender best practices set out the same core requirements and add that unsubscribes must be honoured within 2 days.
RFC 8058 defines one-click unsubscribe precisely: a List-Unsubscribe header with an HTTPS URI, a List-Unsubscribe-Post header with the value List-Unsubscribe=One-Click, and a valid DKIM signature that covers both. The image-to-text, shortener and link-text checks are not in any of these documents; they are A2Z heuristics based on common spam and phishing patterns, and are labelled that way.
How alignment is judged here
DMARC compares the From domain with the DKIM d= domain and the SPF (envelope) domain, usually in relaxed mode, where the two only need the same organisational domain - so mail.northwind.example aligns with northwind.example. This page approximates the organisational domain from the last two labels, or three for common suffixes such as co.uk and com.au; it does not carry the full Public Suffix List. The receiving server's own Authentication-Results header, when present in a received copy, is the authoritative answer and is shown as reported.
Limitations: what the result does not prove
- It does not look up DNS, so it cannot confirm that your SPF record, DKIM public key or DMARC policy exist or pass. Use the linked checkers for that.
- It does not verify DKIM signatures cryptographically; it reads the d= and h= tags only.
- Sender reputation, complaint rates (Google and Yahoo ask for under 0.3%), sending volume and recipient engagement decide most inbox placement, and none of them are visible in a single message.
- Unsubscribe-link detection looks for common wording in several languages and common platform merge tags; an unusual wording may be missed.
- The organisational-domain logic is an approximation without the full Public Suffix List.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- Google - Email sender guidelines - checked 19 Sep 2026
- Yahoo Sender Hub - best practices - checked 19 Sep 2026
- RFC 8058 - One-Click Unsubscribe (List-Unsubscribe-Post) - checked 19 Sep 2026
- RFC 5322 - Internet Message Format - checked 19 Sep 2026
- RFC 7489 - Domain-based Message Authentication, Reporting, and Conformance (DMARC)
- RFC 2369 - The Use of URLs as Meta-Syntax for Core Mail List Commands
Frequently asked questions
What does one-click unsubscribe actually require?
RFC 8058 needs three things together: a List-Unsubscribe header containing an HTTPS URI, a List-Unsubscribe-Post header whose value is exactly List-Unsubscribe=One-Click, and a DKIM signature whose h= tag covers both headers. A mailto address alone, or an http:// address, does not qualify. The receiver sends a POST to the HTTPS address and you must stop mailing that person.
Why does the checker say my DKIM is not aligned?
Your message is signed with a domain whose organisational domain differs from your From domain - typically your email platform's shared domain. Gmail and Yahoo require bulk senders to align the From domain with DKIM or SPF. Most platforms let you set up a custom DKIM domain (sometimes called domain authentication) so the signature uses your own domain.
Is a visible unsubscribe link needed if I have the List-Unsubscribe header?
Yes, for bulk marketing mail to Gmail. Google's sender guidelines ask for both one-click unsubscribe support and a clearly visible unsubscribe link in the message body. The header gives mail apps an unsubscribe button; the link serves readers whose app does not show one.
Should I check the raw message or just my template?
The raw source of a test sent to yourself is better, because your sending platform adds headers at send time - DKIM-Signature, Message-ID, Return-Path, Date and often List-Unsubscribe. A template shows only the content, so header findings will be missing or may be reported as absent when your platform would have added them.
Does the image-to-text check follow a published rule?
No. Neither Google nor Yahoo publishes an image-to-text ratio. The warning is an A2Z heuristic: emails that are mostly one image with little live text are a common spam pattern, and they show nothing useful to readers who block images. The figures are shown so you can decide for yourself.
Why is a 102 KB HTML size a problem?
Email service providers report that Gmail clips messages whose HTML is larger than about 102 KB and shows a View entire message link instead of the rest. The clipped part often includes the footer and the unsubscribe link. Google does not document the exact figure, so treat it as a practical limit rather than a rule.
Can this page tell me if my email will land in spam?
No tool can from one message. Inbox placement depends mostly on your sending reputation, complaint and bounce rates and how recipients engage, which only the mailbox providers see. This checklist removes the avoidable reasons for rejection or distrust before you send; watch Google Postmaster Tools and your platform's reports afterwards.
Last reviewed by the A2Z.Tools team against the sources listed above.
