I tried to send an email, but there was a problem with one of the addresses. I thought about deleting it, but the email did not appear in the Outbox.
Now, I keep receiving error alerts from the em client, but I cannot delete the message because it does not appear in the Outbox, and I do not know how to resolve the problem.
The solution was to make the Outbox visible in Favorites:
Go to Menu → Settings → General → Folders.
Locate the Favorites settings.
Enable Outbox.
Return to the main screen.
Open the Outbox displayed in Favorites and delete the message using Shift + Delete.