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If one mail failed (due to wrong email/else) during schedule process that stuck another last all mails in outbox....if anyone facing this issue please reply for solution.

Questa recensione riguarda una versione precedente (6.3.1) del componente aggiuntivo. 

I agree that this is annoying.

Unfortunately, it's a Thunderbird bug, not a bug in Send Later. I reported it to the maintainers of Thunderbird almost six years ago: https://bug635044.bugzilla.mozilla.org/show_bug.cgi?id=749654 . Unfortunately, they still haven't fixed it.