If a mail message is sent to username@eso.org and the sender believes the address to be right, then here's the order of things to check
First verify if the user does exist in the mail system, but running:
$ telnet web3 smtp
220 web3 sendmail
vrfy username
If the user exists then the problem probably lies with the MUA or the user's typing.
Otherwise, login to poisson and check the user exists in /var/aliases/forward.list. If the user exists then in the same directory run:
poisson# make
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