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The commands using dialog do not work because there is no terminal (tty) associated with the at process. Like cron, at is intended to be a noninteractive environment. Since there is no terminal for dialog to interact with, setting TERM does not help. If you happen to be logged in (and have X running), you can make dialog appear in a separate terminal. ...
Turns out there was an alias defined for root in /etc/mail/aliases and this is where it was coming from. There's no way for me to bypass it per-command with this on, as any mail sent to root's mailbox will automatically get forwarded to that alias.
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