The majority of email bulletins are designed with a fixed width as opposed to a fluid layout. This is primarily because the majority of email clients and web-based email providers do not use the full width of your screen to display an email message.
There are a lot of email client preview panes that are still not suitable for emails exceeding 650px wide. Extra-wide emails (say, exceeding 800px), either introduce awkward scroll bars to the preview pane, or are horizontally truncated.
The width limitation is largely set by the preview panes that feature in desktop clients such as Outlook and Apple Mail. Webmail clients are far more forgiving; with Hotmail and Gmail’s preview panes expanding to the width of the email design.
The practical upside to being limited to a usable width of 550-650px is readability - the wider the email, the more difficult it is to read. Emails become unusable once horizontal scrollbars come into play, so it really does pay to keep your designs slim and trim.
With that said, ClubRunner has designed your Bulletin with a maximum width 600 pixels. This way you will avoid horizontal scrolling in most cases and your readers will see the bulletin(s) as you have designed it.