Hello, I didn't see any other way to report this bug, and the support link from within Opera Mail invited me to search/post on the forums.
Opera Mail isn't being developed anymore so reporting a bug won't help. There's no where to report one for Opera Mail anymore anyway.
For support, you just have these forums (prefix thread titles with "Opera Mail - " please and post the thread in the appropriate Windows, Linux or Mac forum) and the operabrowser reddit and maybe irc.opera.com.