Is there a standard format for HTML-emails? I have noticed that HTML-formatted graphical emails which I receive are the perfect width for the message pane in Lion's Mail app. 

It almost looks as if Apple designed them (it didn't they are from random companies).

Is this a coincidence, or is there a standard? Did Apple play a role in defining it?
HTML emails from companies are usually made using an online HTML builder, such as xact target, Dot mailer, Mail Chimp and the like.

The templates use HTML to create the emails. In the HTML there is code which will render differently in Mail, and outlook, Hotmail, yahoo mail, etc. The suppliers of email software will spend significant amounts of time making sure teh same version is seen in the same way by all email recipients 

The HTML code in a lot of HTML email also defines whether you are using a desktop app, web mail, or even mobile.

The reason your emails fit onto your browser is down to the flood of people using mobile email, on iPhones, iPads, Android devices, etc. The email is built in columns, usually no more that 2. These columns can then be narrow or wide depending on whether the recipient is using a mobile device or a desktop email client.

Next time you get a HTML email have a look at it on your desktop email, and then compare it with how it looks on a mobile.

The hardest mobile to support is Blackberry, as most models do not support HTML, instead they often render a plain text version.

So to finish off and answer your question,

There is no design standard, and Apple did not have a say in it.