Sometimes it can be very handy to include a HTML form in an email campaign. Whether it's a quick customer survey or a subscribe form for another list, they can be a good way to interact with a recipient right there in their email client.
While they can be useful, there are a number of precautions you need to consider before using them.
Hotmail is a no go
Right off the bat, Hotmail will not allow you to include working forms in an email. Worse still, they actually display the form, but remove bits of code required to make it work. By stripping all values from your <form> tag and removing the name values of all form elements, they render your form useless. Your Hotmail recipients can still complete the form, but when they hit the submit button, nothing happens. This is probably worse than the form not displaying in the first place.
Use GET and not POST
Some email clients, most notably Mac Mail do not support forms that use <form method="post" action="http://.... Because of this, you'll need to use the <form method="get" action="http://... approach, which will write all form contents to the query string of the page they are posted to.
Tabbing won't work
Almost all email clients that provide a preview pane, especially popular email clients like Outlook and Outlook Express don't allow you to tab between form elements. This means that the moment a recipient completes the first field in your form and clicks the TAB key, the focus is automatically switched to another part of the software. This has the potential to confuse a lot of recipients and is something to be wary of.
Before deciding whether or not to include a form in your next email campaign, take a close look at your subscriber list. If you notice many Hotmail addresses, make the form available on the web and include a link to the form in your email. If that's not enough to stop you, then considering splitting your subscribers into Hotmail and non-Hotmail and then send a different version to each list. And remember, use GET instead of POST.
Source: Campaign Monitor blog