I installed Opera Mail today and wrote some possible enhancements I am sharing here in this thread.
Do not be bother with this list. It does not say Opera Mail is bad at all. This is simply my notes written before those small problems becomes invisible to me in few days.

Thanks for free-of-charge Opera and Opera Mail products.

Attached file fetch options

I disabled attachment fetch in the first time until I discover that even attachment file names were not visible. It would be great to download attachment file-names if possible. Well, writing this I am thinking it may require much more downloaded bytes maybe.

Search bar

Having a round cross button inside the search bar would help clearing this input filed with the mouse.

Invisible items when tweaking status bar

I tweaked appearance of my status bar. There is one item called "status" which has no content when tweaking appearance. It make it difficult to place other widgets. It would be great to set a foo text in status state during this step.

Storing IMAP password

I refer to not save my password. I uncheck the checkbox the first launch. But I did'nt see it was checked back on next launch and I had my password saved. It would be great to save this popup checkbox last state.

Showing emails with picture faster

When I enable attachment download, my email take longer to be visible. It looks like it is shown only when attachment are locally available. It would be great to show the email body first.

Again thanks for your valuable products.
Anthony.