The Ultimate Mailto Compatibility Guide (2026)

One of the biggest misconceptions about mailto: links is that they work the same everywhere. They don’t.

While the core specification (RFC 6068) is standard, email clients interpret it differently. A link that works perfectly in Apple Mail might fail silently in Outlook or open a blank window in Gmail.

This guide details the critical differences you need to know to ensure your mailto links work for every user.

At a Glance: Key Differences

Feature	Gmail (Web)	Outlook (Desktop)	Apple Mail (iOS/Mac)
Max URL Length	~8,000 chars	~2,000 chars (Critical)	~30,000+ chars
Body Line Breaks	Works (%0A)	Works (%0A)	Works (%0A)
Multiple Recipients	Comma (,)	Semicolon (;) or Comma	Comma (,)
HTML in Body	❌ No	❌ No	❌ No
Attachments	❌ No	❌ No	❌ No

Pro Tip: The biggest “gotcha” is Outlook’s character limit. Always keep your generated mailto links under 2,000 characters to be safe. You can check this automatically with our Mailto Generator.

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Detailed Breakdown by Platform

1. Microsoft Outlook (Desktop & Web)

Outlook is the strictest environment for mailto links.

• The 2,000 Character Limit: Outlook for Windows has a hard limit on command-line arguments. If your mailto link (including “mailto:”, subject, body, etc.) exceeds roughly 2,033 characters, it will fail silently or truncate the message.

• Delimiter Confusion: Older versions of Outlook specifically required semicolons (;) to separate multiple email addresses. Modern versions generally accept commas, but if you are targeting corporate environments with legacy software, this can still be an issue.

2. Gmail (Web Client)

When Gmail is set as the default handler in Chrome/Edge:

• Encoding is Critical: Gmail is very sensitive to URL encoding. If you have special characters (like & or ?) in your body text that aren’t properly encoded as %26 or %3F, Gmail may cut off the message at that point.

• Window Behavior: Gmail typically opens in a new browser tab or a pop-up window. Ensure you don’t use target="_blank" on your mailto links, as this can sometimes trigger a popup blocker or leave a blank page open.

3. Apple Mail (iOS & macOS)

Apple Mail is generally the most compliant and robust client.

• Mobile Behavior (iOS): On iPhones, clicking a mailto link usually slides up a “New Message” sheet without leaving your app or website. This provides the smoothest user experience.

• “Click to Nothing”: If a user has deleted the default Mail app on iOS and hasn’t set a replacement (like Gmail for iOS), clicking the link will do nothing. This is an OS-level limitation.

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Best Practices for Maximum Compatibility

To make your links bulletproof, follow these rules:

1. The “Safety Zone” for Length

Aim for a maximum of 1,500 characters. This gives you a safe buffer below Outlook’s limit. If you need to send a longer message, consider using a contact form instead.

2. Always Encode EVERYTHING

Never write raw spaces or special characters in your HTML.

• Bad: href="mailto:me@test.com?subject=Hello World"
• Good: href="mailto:me@test.com?subject=Hello%20World"

Our generator handles this automatically for you.

3. Use Bcc for Multiple Recipients

If you are adding 5+ recipients, putting them in the to field often triggers spam filters or privacy concerns. Use bcc instead.

• Compatibility Note: bcc is supported by virtually all modern clients.

4. Don’t Rely on “From”

You cannot set the from field. The sender is always determined by the user’s email client configuration. Any from= parameter in your URL will be ignored.

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Troubleshooting Checklist

If a user reports your link isn’t working:

1. Check Default Client: Ask if they have an email app installed and set as default. (Common issue on fresh Windows installs).
2. Check Length: Is the generated link over 2,000 chars?
3. Check Encoding: Did you manually type the link? Use a tool to generate it to ensure valid percent-encoding.

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Summary

Goal	Recommendation
Safe Length	< 2,000 characters
Line Breaks	Use %0A
Separators	Use Commas (,)
Testing	Test on Outlook (Win) & iOS Mail

Ready to create a compliant link? Use our Free Mailto Generator to build links that work everywhere.