Hello,
How to enable html view by default? I have to double click on top on every email to see it in html...
HTML DEFAULT VIEW
Re: HTML DEFAULT VIEW
The answer to your question can be found HERE, which involves the POP Peeper "Smart HTML Viewing" feature.
To clarify the link provided above; you presumably have this setting enabled in "Tools" > "Options" > "View Messages" tab which is recommended as the safest approach. With this setting enabled as illustrated, if you consider it to be safe to display a specific message in HTML then by clicking the "Legit" button for the message this specific sender will no longer display the warning banner you mentioned and instead display messages in HTML for that sender from that point forward.
(additional details are provided in the link posted above)
To clarify the link provided above; you presumably have this setting enabled in "Tools" > "Options" > "View Messages" tab which is recommended as the safest approach. With this setting enabled as illustrated, if you consider it to be safe to display a specific message in HTML then by clicking the "Legit" button for the message this specific sender will no longer display the warning banner you mentioned and instead display messages in HTML for that sender from that point forward.

Good judgment comes from experience and a lot of that comes from bad judgment. - Will Rogers
Re: HTML DEFAULT VIEW
Follow-up point of reference; to clarify, the "Legit" status for a message (sender) can be toggled on/off via the "Legit" button when either selecting the message in the POP Peeper inbox or when viewing the message: 
Alternatively (not using the "Legit" button) by right-clicking on a message in the main POP interface inbox the "Legit" status can also be toggled as illustrated below involving the "Mark as legitimate" setting (checked is "Legit", unchecked is not "Legit"): ... I thought I'd mention this just in case you might wonder how to undo a message (sender) that has been marked as "Legit". 
Good judgment comes from experience and a lot of that comes from bad judgment. - Will Rogers