Understanding Email Client Rendering Differences
Gmail, Outlook, and Apple Mail all render HTML email differently. Learn the common pitfalls and how to ensure your campaigns look great everywhere.
Email Preheader Text: The Hidden Conversion Booster
What Is Preheader Text?
When an email arrives in your inbox, you see three pieces of information before deciding whether to open it:
- The sender name — who it's from
- The subject line — the headline
- The preheader — a short preview of the email body
The preheader (also called preview text) appears in the inbox list view next to or below the subject line. Most email clients display 40–120 characters, depending on the device and app.
Here's how it appears in a typical inbox:
From: Acme Newsletter
Subject: Your free resource is waiting
Preview: Download the complete guide to email automation — no sign-up needed.
Without a custom preheader, email clients pull the first text they find in your email's HTML — often something unhelpful like "View this email in your browser" or ---.
How to Set Preheader Text in AcelleMail
In AcelleMail's campaign editor:
- Open your campaign and go to the Email Content step
- Locate the Preheader Text field (directly below the subject line)
- Enter your preheader — AcelleMail automatically injects it as invisible text at the top of your HTML
Alternatively, if you're using a custom HTML template, add this snippet immediately after the opening <body> tag:
<span style="display:none; max-height:0; overflow:hidden; mso-hide:all;">
Your preheader text goes here. Fill with enough text to reach 100 characters so trailing content doesn't bleed through.
</span>
The trailing filler text trick prevents your actual email content from appearing after the preheader in the preview pane.
Preheader Length Guide by Client
| Email Client | Visible characters (approx.) |
|---|---|
| Gmail (desktop) | 100–140 |
| Gmail (mobile) | 40–70 |
| Apple Mail (desktop) | 140+ |
| iOS Mail | 90–140 |
| Outlook (desktop) | 50–70 |
Write your most important preheader content in the first 50 characters — that's what every client will show.
Writing High-Converting Preheaders
Complement the subject line, don't repeat it:
| Subject | Weak preheader | Strong preheader |
|---|---|---|
| "Your order has shipped" | "Your order has shipped" | "Estimated delivery: Friday. Track it here." |
| "50% off ends tonight" | "50% off sale ends tonight" | "Use code SAVE50 at checkout — midnight deadline." |
| "New blog post: SEO tips" | "Check out our latest blog post" | "How we ranked #1 in 60 days — no paid ads." |
Techniques that work:
- Answer the implicit question — "Why should I open this?"
- Create urgency or specificity — Numbers, deadlines, exclusive details
- Use the preview as a second headline — Some readers make open decisions from subject + preheader alone
- Ask a question — "Have you tried this yet?" paired with an intriguing subject
- Reveal a benefit — What will they get if they open?
What Not to Do
- Do not repeat the subject line word-for-word
- Do not leave it blank (clients will pull random content)
- Do not stuff it with keywords — write for humans, not algorithms
- Do not exceed 140 characters without filler text (trailing content bleeds into preview)
Testing Your Preheader
Before sending, always preview your email in AcelleMail:
- Go to Preview → Inbox Preview
- Check how the subject + preheader combination looks at mobile width
- Send a test email to your own inbox and check Gmail, iOS Mail, and Outlook
The preheader is one of the fastest, lowest-effort improvements you can make to your email program. A well-written preheader reliably lifts open rates by 5–15% without touching anything else in the campaign.
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