Apple's Mail app has been buggy for quite a while now, actually. One other bug is that it keeps automatically fetching e-mails even when it is set to "Manually" in Settings.
Anyway, I noticed this "message cannot be displayed" issue as well, basically only on e-mails that I send to myself from my iPhone or iPad. The bug is still present in iOS / iPad OS 14.4.
When you send an e-mail to yourself via the Mail app, you essentially end up with 2 copies of this e-mail. One arrives into your "Inbox" folder and the other copy stays in your "Sent" folder. The Mail app doesn't seem to correctly switch between the two copies when tapping between them. This might be what is contributing to the randomness of the e-mail being "readable" or "bad".
Anyway, here's what I found out in my testing (it gets a bit complex):
After you send yourself an e-mail from the Mail app, whether the received e-mail is "readable" or "bad" depends on where you open it first.
1- If you tap on the e-mail in the Inbox folder first, it will be random (so it seems).
- If it is "bad" in Inbox, the copy in Sent is also "bad". If you delete the copy in Sent, the one in Inbox will become "readable".
- If it is "readable" in Inbox, the copy in Sent is also "readable". If you delete the copy in Sent, the one in Inbox remains "readable".
2- If you tap on the e-mail in the Sent folder first, it will always (so it seems) be "readable". If you then tap on it in the Inbox folder, it will be "readable" there as well.
- If you delete it from Inbox, the copy in Sent will become "bad".
- If you delete it from Sent, the copy in Inbox will remain "readable".
So basically the bottom line is this:
If you send an e-mail to yourself and it ends up showing "message cannot be displayed" when you open it in Inbox, go to the Sent folder and delete the copy in there. That should make the one in Inbox readable.
