The hypothetical mail-sending process would presumably want to send mail and then delete the associated record from the table of pending mails ... so what if it fails after sending the mail and before committing the delete?
What this does do for you is replace the risk of phantom emails (mail sent but corresponding action inside the database never committed) with the risk of duplicate emails (mail-sender sends you another one after it restarts). In most cases I think I'd prefer the latter.
You have this possibility anyway. If a mailserver thinks it has failed to forward the message, it will resend. There is always a small window where the receiving mailserver might actually have received the message without the acknowledgement being logged by the sender.
-- Richard Huxton Archonet Ltd
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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match
