Cannot sent message via webmail: “User unknown in virtual alias table”

If Postfix files are corrupted completely, or empty:

  1. Create a backup of the current configuration:
    ~# cp -Rp /var/spool/postfix/plesk /var/spool/postfix/plesk.backup
  2. Remove redundant records from the Postfix database:
    ~# cd /var/spool/postfix/plesk/
    ~# for db_file in virtual virtual_domains ; do
        ls -1 --color=never /var/qmail/mailnames > /tmp/domains.txt
        hostname -f >> /tmp/domains.txt
        postmap -s /var/spool/postfix/plesk/$db_file > /tmp/${db_file}_temp
        grep -f /tmp/domains.txt /tmp/${db_file}_temp > /tmp/$db_file
        postmap /tmp/$db_file
        mv /tmp/$db_file.db /var/spool/postfix/plesk/
  3. #set locale after mchk
    export LANG=”en_US.UTF-8″
    export LC_ALL=”en_US.UTF-8″
  4. /usr/local/psa/admin/sbin/mchk –with-spam

more info