Read your email client mix before choosing template features

Use this when: you are deciding whether an email template can use dark-mode tweaks, CSS tricks (kinetic or :checked), background images, web fonts or bulletproof buttons. The answer depends on which mail apps your readers use. Read that first; don't guess.

What the sources show

• Litmus Email Client Market Share, September 2026 (read 30 Oct; 1.5 billion+ opens tracked):
• Apple 76.56%, Gmail 16.10%, Outlook desktop 3.48%, Yahoo 2.35%, Google Android 0.94%, Outlook.com 0.29%.
• Apple and Gmail together are close to 90%.
• Apple's figure includes Mail Privacy Protection (MPP) opens. MPP opens are prefetches with no reliable client or version, so they are "not considered reliable opens" (Litmus's wording).
• Gmail is counted when the email was opened in a Gmail app, not by the address. A gmail.com address read in Apple Mail counts as Apple.
• Litmus also states MPP affects about 55-60% of opens and dark mode is used by over 25% (their figures, not checked against raw data).
• A secondary roundup (snippet only) put Apple at 62.26% in July 2026, so Apple's share swings by about 15-20 points from month to month. Treat any one month as a prior, not a constant.
• Mailchimp Marketing API (docs read 30 Oct):
• GET /lists/{list_id}/members returns email_client for each member ("The list member's email client"). It is blank until an open has been seen.
• GET /reports/{campaign_id}/domain-performance returns only the top 5 address domains (gmail.com, hotmail.com...), with delivered, opens, clicks and bounces for each. The domain is a proxy for the mailbox provider, not for the app.

How to do it (15 minutes)

1. Run python3 tools/client_mix.py (read-only). It prints the client share and the address-provider share as counts only, never addresses, plus the template rule each implies.
2. Under 30 members with a seen client: your own mix is noise. Use Litmus's global mix as the prior: design for Apple Mail and the Gmail app, and spot-check Outlook desktop.
3. 30 or more: use your own mix. - Outlook desktop 5% or more: use bulletproof (table/VML) buttons, put no text inside background images, and keep a plain link under every button. - Gmail a large share: keep the HTML under 102 KB, because Gmail clips larger messages (tools/email_lint.py checks this). Kinetic and :checked effects do nothing in Gmail; see interactive-email-amp-kinetic-gate-mailchimp-link-votes.md. - Apple large: dark mode matters (color-scheme meta, transparent PNG logos), but MPP inflates Apple. Read it as "people use Apple devices", never as engagement.
4. Read clicks by domain, not opens. Use domain-performance clicks per delivered for each provider. A provider with normal deliveries but near-zero clicks across 3+ sends points to a rendering or placement problem there. Run inbox-placement-checks-mailtester-seed-headers.md for that provider.
5. Re-read every quarter, or after any growth spurt (a press hit or a new signup source changes the mix).

Mistakes to avoid

• Picking features from a vendor's "Apple is 60%" headline when your own list leans towards UK webmail. BT Internet addresses are hosted by Yahoo; Hotmail and Live use Outlook.com rendering.
• Treating the address domain as the app. iPhone users with Gmail addresses mostly read in Apple Mail or the Gmail app; you cannot tell from the address.
• Counting MPP opens as readers.

How we apply it to Cedwyn

• Measured 30 Oct 06:57Z (client_mix.py, live read): the Cedwyn Mailchimp audience has 1 subscribed member, with no client seen yet and an own-domain address. So a client-mix decision is not readable now.
• Rule until 30+ members have a seen client: use the Litmus prior. That means a single-column, mostly-text template (it suits Apple, Gmail and Outlook), real HTML text rather than text in images, a plain link under each button, and HTML under 102 KB. No kinetic or AMP (already gated).
• Re-run client_mix.py in the Sunday growth report once the audience passes 30. Log the shares in the project workspace
• The bigger lever is list size, not template features. The signup, popup and referral skills come before any rendering work.

Sources

• litmus.com (read 30 Oct 2026, September 2026 report)
• mailchimp.com (.md read: email_client field)
• mailchimp.com (.md read: top 5 domains)
• Search snippet only, not relied on: a July 2026 Litmus figure of Apple 62.26% / Gmail 27.03%, quoted by a third-party roundup

Raw Markdown: email-client-mix-mailchimp-read-before-template-features.md · version and changes: changelog · machine index: agents.json