You can create plain text and HTML versions of your messages to send to your subscribers. There are compelling reasons to use either or both. Once you decide which version to use:

For Plain Text Messages:

We recommend creating your plain text messages in Notepad, then copying and pasting them into the plain text section of your message.

For the HTML:

You have several choices:

Use one of our HTML design templates; or

Create your message in our HTML editor by copying/pasting from a text editor or typing directly in it; or

Create your message in an external HTML editor (such as Dreamweaver) then copy and paste that HTML into a message you create in your account

Please do not use Microsoft Word or other word processing programs to create your messages. Copying content from those programs causes formatting issues, as they contain proprietary formatting that will not render properly in an HTML email message.

If you are going to create your message outside of AWeber and then paste it in, use Notepad or another plain text editor that will not insert proprietary formatting.

source: Aweber.com