If you were using PP to send the email, I would recommend using the "forward as attachment" method, which would retain the HTML format. POP Peeper doesn't currently support HTML in sending, so that's really the only way to do that right now.
As for sending to an external email client, I think that the best way will be to open the message in TB and then use it to reply. However, POP Peeper does provide a way that makes this more convenient compared to what you're probably doing now:
When you view a message in PP, under the "File" menu is "Open Message in Client"; assuming you've set the email client to TB in PP, this will open the message. That's the first major step to making this more convenient (as opposed to having to open TB, download the messages, find the right message and open it)
But another way to make this more convenient is to add the "Open message in client" to the toolbar. There's not a default button, which means you'll have to use the "add your own button" method as described here:
https://www.esumsoft.com/pop-peeper/ref ... tom-icons/
The ID for "open email client" is 33423, so the filename of the bitmap would be:
Msg_33423_Client.bmp
(you can change the "client" text to whatever, but it should be short else the button will be very large)
Let me know if that's something you think would be helpful or not, or if you need any further help with creating the toolbar button.