I just started using Mail 1.0, Build 1040. When I hide the mailing list panel on the left side and then exit Opera Mail, it reappears after I open Mail again.

That's intentional. If Opera detects a new mailing list or a new message in a mailing list, it'll unhide the "Mailing Lists" access point. There's no way to turn that off.

You can right-click on the "Mailing Lists" access point header, goto "customize" and uncheck the mailing lists you want. But, I think they'll unhide on you too.

Instead, just left-click the "Mailing Lists" access point header to collapse it. Or, better yet, right-click in the "Mailing Lists" access point somewhere and create a folder named "hide" (for example). Then, drag the mailing views you want into it. Then, just keep the "hide" folder collapsed.

Now, as for the mail panel itself, if you get to mail by "Menu -> Mail -> Read Mail" or "Menu -> Feeds -> Read Feeds", Opera will do some automatic opening and closing of the panel list (the mail panel if it's selected i the panel list and expanded) depending on what you do.

To turn that off, you can close down Opera and edit operaprefs.ini in the preferences folder (see Menu -> help -> about Opera Mail for the location). Under the "User Prefs" section, set:

Auto Mail Panel Toggle=0

Or, instead of getting to mail via the menu, just press F4 or click the panel list toggle at the bottom left of Opera on the status bar. Or, if you close down Opera Mail and add:

Show panel toggle=1

under "User Prefs" to operaprefs.ini, they're be a thin strip at the left of Opera's windows where clicking it will open/close the panel list (and the mail panel specifically if it's selected in the panel list).

On a side, if you need to reset the mail panel, click the wrench icon on the mail panel toolbar and choose reset.