I have mine set so sent messages are explicitly saved by tb to a folder (originally gmail's sent folder and now to local folders) and I see the failed message there (after a a few minutes). So maybe to not lose the messages that don't send due to gmail failure you need to set tb to save sent messages somewhere? Seems like this might save them twice (once by gmail and then by tb) but I only ever see one saved per sent message.
Bug 1556269 Comment 9 Edit History
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I have mine set so sent messages are explicitly saved by tb to a folder (originally gmail's sent folder and now to local folders) and I see the failed message there (after a few minutes). So maybe to not lose the messages that don't send due to gmail failure you need to set tb to save sent messages somewhere? Seems like this might save them twice (once by gmail and then by tb) but I only ever see one saved per sent message.