Effective maximum mailto: body lengths

Question

There is a maximum length for the text in the &body section of a mailto: link. According to one of my co-workers, the W3C publish the limit as 256 (I don't have a link to back this up, though).

We're embedding mailto: links in both an e-mail and a webpage and have successfully used more than 256 characters. After a certain point, though, e-mail clients and browsers start flaking out and refusing to open the link.

I would like to know the actual maximum lengths allowed for the following:

Mail clients:

• Outlook (2003, 2007, 2010)
• Eudora (7.1, and/or whatever the latest version is)
• Thunderbird (latest version)

Browsers:

• Firefox (3+)
• IE (6, 7, 8)
• iPhone browser

Any and all numbers you can provide will be gratefully received.

Answer 1

The standard doesn't define a maximum length, leaving implementation up to browsers and mail clients (See IETF RFC 2368).

Microsoft products do have set limits:

• IE GET limit is 2,083 http://support.microsoft.com/kb/208427
• Outlook express: 456 characters http://support.microsoft.com/kb/q182985/

Other browsers are likely to work up to lengths beyond that of a reasonable email body. The iPhone doesn't have a documented limit, but works with up to 1MB of text.

Modern browsers that support data urls (everything except IE<9) should be fine.

Answer 2

For browsers with JS consoles, an easy test:

for (var i=2014; i>1600; i--) {var good=1; try {location.href='mailto:?body='+'a'.repeat(i)} catch (e) {good=0;} if (good==1) {console.log(i+13);break;}}

(The 13 is for the length of mailto:?body=.)

On Firefox 32.0.3 this produces 2008 (body length 1995). On Thunderbird 31.2.0, all 1,995 characters make it into the body of the new e-mail.

Some say it's mainly OS-setting-dependent but on Windows, at least, I couldn't find any registry entry related to mailto with a number near 2,000.