This is what comes up after [Cancel] in the first dialog. It says: "The message was sent [which seems to be true, based on the replies I get], but no copy could be saved in the Send folder (Inbox), because there was a problem while accessing it. You can try again, or save the msg in Local Folders/Inbox-MyAccount. When I click [Save], the dialog goes away. Now while I read the message carefully, I see that indeed, there is a new folder in Local Folders, which contains my mail. At least the mail is still there. I was looking in Drafts before. I didn't expect it to create new folders on its own, with non-standard naming scheme.
Bug 1868160 Comment 10 Edit History
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This is what comes up after [Cancel] in the first dialog. It says: "The message was sent [which seems to be true, based on the replies I get], but no copy could be saved in the Send folder (Inbox), because there was a problem while accessing it. You can try again, or save the msg in Local Folders/Inbox-MyAccount. When I click [Save], the dialog goes away. Now while I read this excessively long message carefully, I see that indeed, there is a new folder in Local Folders, which contains my mail. At least the mail is still there. I was looking in Drafts before. I didn't expect it to create new folders on its own, with non-standard naming scheme.