Another standard feature missing in Opera's built-in mail client is support for specifying an IMAP drafts folder.
Most other mail clients allow you to specify an IMAP drafts folder. Then, either you manually choose to upload a snapshot of the message you're composing to the drafts folder on the IMAP server or it's triggered automatically by message change listeners and or at a specified interval (1 minute for example). Then, you can continue editing the draft later, even in othe mail clients since the draft is on the server.
Now, Opera does support automatic creation of a draft when you're composing a message where you can continue composing later (in case of a crash etc. or you just closed the compose tab without discarding the draft because you want to continue later). It's just that the draft is only local and won't be seen by other mail clients since the draft isn't on the server. Having the most recent copy on the server is the important part.
To work around this in Opera, after the draft is created, you have to copy it to the Drafts IMAP folder view to upload a copy to the server. You do this by ctrl + dragging (ctrl not needed in the case, but do it anyway) from "All Messages/Drafts" to the Drafts IMAP folder view under the "email@example.com" IMAP folder access point. But, if the draft is updated, you have to delete the copy on the server and copy the draft to the Drafts IMAP folder view again to reupload it. But, even if you do all that, if you edit the draft in another client, the local copy in "All Messages/Drafts" in Opera won't be updated (or removed if the message was completed and sent) out.
This is a basic feature that all IMAP clients need to support.
Now, Opera recently (and finally after all those years) added support for specifying IMAP trash and spam folders. So, hopefully Opera will get support for specifying an IMAP drafts folder too.