What happens when a recipient opens an HTML email on a mail client that do not fully support HTML formatting?
It's important to note not all clients provide full support for HTML used mail designs. If a recipient is using a client that does not fully support the HTML syntax used in your email, the mail content may lose its original layout or formatting. Images might be attached to the mail and not shown inline. HTML source code markup syntax might be presented.

Although result may differ with different formatting, HTML mail has been tested to render relatively well with mail clients such as Mail on the iPhone and iPad, MS Outlook, Thunderbird and Mail on Mac.

Known clients that did not appear to render the mail as well include MS Outlook on Windows, the web version of Gmail and the web version of Yahoo! Mail.

If you have a large subscriber list, we recommend surveying the users on their email preferences, and providing options for receiving a copy of the email in either HTML or plain text.