Tente o seguinte:
join -t: cleartext.txt maillist.txt | awk -F: '{ print "echo Username: "$1" Passwd: "$2" | mailx -s Information "$3 }' |sh
Se várias linhas forem necessárias no corpo do email, consulte abaixo
#!/bin/bash
>tmp.txt
join -t: cleartext.txt maillist.txt | while read line
do
echo $line
USER=$(echo $line | awk -F: '{ print $1 }')
PSWD=$(echo $line | awk -F: '{ print $2 }')
EMAILID=$(echo $line | awk -F: '{ print $3 }')
echo "Hello $(echo $USER | tr a-z A-Z), your password has been changed!!!<br>" >tmp.txt
echo "<b>Username</b>: $USER<br>" >>tmp.txt
echo "<b>Password</b>: $PSWD<br>" >> tmp.txt
cat tmp.txt | mailx -s "$(echo -e 'Password Changed!!!\nContent-Type: text/html')" $EMAILID
done