环境：Ubuntu18.04
问题说明：在按照配置教程的步骤安装iredmail0.9.9过程中，到所有设置都完成以后，报错：

Reading package lists... Done
[ INFO ] Installing package(s): postfix postfix-pcre libsasl2-modules postgresql postgresql-client postgresql-contrib postfix-pgsql libdbd-pg-perl php-cli php-fpm php-json php-gd php-curl mcrypt php-intl php-xml php-mbstring php-pgsql nginx-full dovecot-imapd dovecot-pop3d dovecot-lmtpd dovecot-managesieved dovecot-sieve dovecot-pgsql amavisd-new libcrypt-openssl-rsa-perl libmail-dkim-perl clamav-freshclam clamav-daemon spamassassin altermime arj nomarch cpio lzop cabextract p7zip-full rpm ripole libmail-spf-perl unrar-free pax lrzip mlmmj sogo sogo-activesync sope4.9-gdl1-postgresql python-sqlalchemy python-dnspython python-psycopg2 python-pymysql python-jinja2 python-netifaces python-webpy python-beautifulsoup python-lxml python-pycurl python-requests uwsgi uwsgi-plugin-python python-bcrypt fail2ban zlib1g libuuid1 libmnl0 curl lm-sensors netcat bzip2 acl patch cron tofrodos logwatch unzip bsdutils liblz4-tool
Reading package lists...
Building dependency tree...
Reading state information...
Package unzip is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package php-pgsql is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package php-cli is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package postgresql is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package amavisd-new is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package postfix is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  postfix-sqlite postfix-lmdb

Package spamassassin is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package clamav-daemon is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package clamav-freshclam is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-jinja2 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package php-curl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-sqlalchemy is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-lxml is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package postgresql-client is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package postgresql-contrib is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package php-gd is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-psycopg2 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-requests is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-netifaces is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-dnspython is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package dovecot-imapd is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package libmail-spf-perl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-pycurl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package libmail-dkim-perl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package pax is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package postfix-pgsql is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-pymysql is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package libcrypt-openssl-rsa-perl is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package python-bcrypt is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

Package dovecot-pop3d is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'postfix' has no installation candidate
E: Unable to locate package postfix-pcre
E: Package 'postgresql' has no installation candidate
E: Package 'postgresql-client' has no installation candidate
E: Package 'postgresql-contrib' has no installation candidate
E: Package 'postfix-pgsql' has no installation candidate
E: Package 'php-cli' has no installation candidate
E: Package 'php-gd' has no installation candidate
E: Package 'php-curl' has no installation candidate
E: Package 'php-pgsql' has no installation candidate
E: Package 'dovecot-imapd' has no installation candidate
E: Package 'dovecot-pop3d' has no installation candidate
E: Unable to locate package dovecot-managesieved
E: Unable to locate package dovecot-sieve
E: Package 'amavisd-new' has no installation candidate
E: Package 'libcrypt-openssl-rsa-perl' has no installation candidate
E: Package 'libmail-dkim-perl' has no installation candidate
E: Package 'clamav-freshclam' has no installation candidate
E: Package 'clamav-daemon' has no installation candidate
E: Package 'spamassassin' has no installation candidate
E: Package 'libmail-spf-perl' has no installation candidate
E: Package 'pax' has no installation candidate
E: Package 'python-sqlalchemy' has no installation candidate
E: Package 'python-dnspython' has no installation candidate
E: Package 'python-psycopg2' has no installation candidate
E: Package 'python-pymysql' has no installation candidate
E: Package 'python-jinja2' has no installation candidate
E: Package 'python-netifaces' has no installation candidate
E: Package 'python-lxml' has no installation candidate
E: Package 'python-pycurl' has no installation candidate
E: Package 'python-requests' has no installation candidate
E: Package 'python-bcrypt' has no installation candidate
E: Unable to locate package logwatch
E: Package 'unzip' has no installation candidate
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