A URL (Universal Resource Locator) is what produces a link, whether it's on a web page or in an email. That URL tells the link where to go.
A file:// link points to something on your computer. In order for a link to work in email, it needs to point somewhere on the Internet. If you sent the message with a file:// link, none of your subscribers would be able to click on that link, so the composer produces an error, and won't send the message.
To figure out where your file:// link is located, click the button at the end of the third row of buttons (that says HTML). This will show you the HTML source of your message. Then click in that box, and select "Find" in your browser's menu. Type "file:", and it will show you which of your links is a "file://" link.
You need to correct that link before you can send the message. To correct the link, you need to point the link to a URL that starts with "http://". Make sure your link can be opened in your web browser.
If you still need assistance, please contact customer service.
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