The current version is not supported or Postfix is not installed successfully：

aaP_carlos_lizano
Hi, is there any error during installation? It can be seen here.
Can you provide us with your aapanel information if it is convenient? If so, please send it to 1096308852@qq.com, thank you.

aaPanel_Kern
Recently, I did the modification on the /www/server/panel/install/mail_sys.sh file, adding new functionality for the Centos 9 (which I'm using CentOS Stream 9). Here is my edits:

install_rspamd()
{
  if [[ $systemver = "7" ]];then
    wget -O /etc/yum.repos.d/rspamd.repo https://rspamd.com/rpm-stable/centos-7/rspamd.repo
    rpm --import https://rspamd.com/rpm-stable/gpg.key
    # yum makecache -y
    yum install rspamd -y
  elif [ $systemver = "8" ]; then
    wget -O /etc/yum.repos.d/rspamd.repo https://rspamd.com/rpm-stable/centos-8/rspamd.repo
    rpm --import https://rspamd.com/rpm-stable/gpg.key
    # yum makecache -y
    yum install rspamd -y
  elif [ $systemver = "9" ]; then
    wget -O /etc/yum.repos.d/rspamd.repo https://rspamd.com/rpm-stable/centos-9/rspamd.repo
    rpm --import https://rspamd.com/rpm-stable/gpg.key
    # yum makecache -y
    yum install rspamd -y
  else
    *rest of the code blocks
}
Install_rspamd_rpm() {
    if [[ $systemver = "7" ]]; then
        wget $download_Url/src/rspamd-3.4-1.x86_64.rpm
        rpm -ivh rspamd-3.4-1.x86_64.rpm
        rm -f rspamd-3.4-1.x86_64.rpm
    elif [[ $systemver = "8" ]]; then
        wget $download_Url/src/rspamd-3.4-1.x86_64.rpm
        rpm -ivh rspamd-3.4-1.x86_64.rpm
        rm -f rspamd-3.4-1.x86_64.rpm
    elif [[ $systemver = "9" ]]; then
        wget https://rspamd.com/rpm-stable/centos-9/x86_64/rspamd-3.10.0-1.el9.x86_64.rpm
        rpm -ivh rspamd-3.10.0-1.el9.x86_64.rpm
        rm -f rspamd-3.10.0-1.el9.x86_64.rpm
    else
        *rest of the code blocks
}

download_Url9=http://mirror.ghettoforge.org
download_official_url=https://mirror.stream.centos.org

Install_centos9() {
  # Enable EPEL repository for CentOS Stream 9
  dnf install epel-release -y

  # Remove system-provided postfix if necessary
  if [[ $cpu_arch = "x86_64" && $postfixver != "3.9.0" ]]; then
    dnf remove postfix -y
    rm -rf /etc/postfix
  fi
  # 安装postfix和postfix-sqlite
  wget -O /tmp/postfix3-3.9.0-1.gf.el9.x86_64.rpm $download_Url9/distributions/gf/el/9/plus/x86_64/postfix3-3.9.0-1.gf.el9.x86_64.rpm
  dnf localinstall /tmp/postfix3-3.9.0-1.gf.el9.x86_64.rpm -y
  wget -O /tmp/postfix3-sqlite-3.9.0-1.gf.el9.x86_64.rpm $download_Url9/distributions/gf/el/9/plus/x86_64/postfix3-sqlite-3.9.0-1.gf.el9.x86_64.rpm
  dnf localinstall /tmp/postfix3-sqlite-3.9.0-1.gf.el9.x86_64.rpm -y
  wget -O /tmp/postfix-perl-scripts-3.5.25-1.el9.x86_64.rpm $download_official_url/9-stream/AppStream/x86_64/os/Packages/postfix-perl-scripts-3.5.25-1.el9.x86_64.rpm
  dnf localinstall /tmp/postfix-perl-scripts-3.5.25-1.el9.x86_64.rpm -y
  if [[ ! -f "/usr/sbin/postfix" ]]; then
    dnf install postfix -y
    dnf install postfix-sqlite -y
  fi
  # 安装dovecot和dovecot-sieve
  dnf install dovecot-pigeonhole -y
#  wget -O /tmp/dovecot23-2.3.20-3.gf.el9.x86_64.rpm $download_Url9/distributions/gf/el/9/plus/x86_64/dovecot23-2.3.20-3.gf.el9.x86_64.rpm
#  yum localinstall /tmp/dovecot23-2.3.20-3.gf.el9.x86_64.rpm -y
  if [[ ! -f "/usr/sbin/dovecot" ]]; then
    dnf install dovecot -y
  fi
  # 安装opendkim
#  安装rspamd
  if [ $ping_url != "200" ]; then
      Install_rspamd_rpm
  else
        install_rspamd
  fi
  dnf install cyrus-sasl-plain cyrus-sasl-lib cyrus-sasl libsodium libwins -y
}
...
Install()
{
  if [ ! -d /www/server/panel/plugin/mail_sys ];then
    mkdir -p $pluginPath
    mkdir -p $pluginStaticPath

  # 调整顺序
  filesize=`ls -l /etc/dovecot/dh.pem | awk '{print $5}'`
  echo $filesize

  if [ ! -f "/etc/dovecot/dh.pem" ] || [ $filesize -lt 300 ]; then
    openssl dhparam 2048 > /etc/dovecot/dh.pem
  fi

    if [[ $systemver = "7" ]]; then
      Install_centos7
    elif [[ $systemver = "8" ]]; then
      Install_centos8
    elif [[ $systemver = "9" ]]; then
      Install_centos9
    else
      Install_ubuntu
    fi
  fi
 *rest of the code blocks
}
Uninstall()
{
  if [[ $systemver = "7" ]];then
    yum remove postfix -y
    yum remove dovecot -y
    yum remove opendkim -y
    yum remove rspamd -y
    yum remove dovecot-pigeonhole -y
  elif [ $systemver = "8" ]; then
    yum remove postfix -y
    yum remove dovecot -y
    yum remove opendkim -y
    yum remove rspamd -y
    yum remove dovecot-pigeonhole -y
  elif [ $systemver = "9" ]; then
    yum remove postfix -y
    yum remove dovecot -y
    yum remove opendkim -y
    yum remove rspamd -y
    yum remove dovecot-pigeonhole -y
  else
    *rest of the code blocks
}

Or you can manually install using these commands:

# sudo dnf remove postfix -y
# rm -rf /etc/postfix
# wget -O /tmp/postfix3-3.9.0-1.gf.el9.x86_64.rpm http://mirror.ghettoforge.org/distributions/gf/el/9/plus/x86_64/postfix3-3.9.0-1.gf.el9.x86_64.rpm
# dnf localinstall /tmp/postfix3-3.9.0-1.gf.el9.x86_64.rpm -y
# wget -O /tmp/postfix3-sqlite-3.9.0-1.gf.el9.x86_64.rpm http://mirror.ghettoforge.org/distributions/gf/el/9/plus/x86_64/postfix3-sqlite-3.9.0-1.gf.el9.x86_64.rpm
# dnf localinstall /tmp/postfix3-sqlite-3.9.0-1.gf.el9.x86_64.rpm -y
# wget -O /tmp/postfix-perl-scripts-3.5.25-1.el9.x86_64.rpm https://mirror.stream.centos.org/9-stream/AppStream/x86_64/os/Packages/postfix-perl-scripts-3.5.25-1.el9.x86_64.rpm
# dnf localinstall /tmp/postfix-perl-scripts-3.5.25-1.el9.x86_64.rpm -y
# wget -O /tmp/cyrus-sasl-2.1.27-21.el9.x86_64.rpm https://mirror.stream.centos.org/9-stream/AppStream/x86_64/os/Packages/cyrus-sasl-2.1.27-21.el9.x86_64.rpm
# dnf localinstall /tmp/cyrus-sasl-2.1.27-21.el9.x86_64.rpm
# wget -O /tmp/cyrus-sasl-lib-2.1.27-21.el9.x86_64.rpm https://mirror.stream.centos.org/9-stream/AppStream/x86_64/os/Packages/cyrus-sasl-lib-2.1.27-21.el9.x86_64.rpm
# dnf localinstall /tmp/cyrus-sasl-lib-2.1.27-21.el9.x86_64.rpm
# wget -O /tmp/cyrus-sasl-plain-2.1.27-21.el9.x86_64.rpm https://mirror.stream.centos.org/9-stream/AppStream/x86_64/os/Packages/cyrus-sasl-plain-2.1.27-21.el9.x86_64.rpm
# dnf localinstall /tmp/cyrus-sasl-plain-2.1.27-21.el9.x86_64.rpm

Then, initialize the mail server environment prompt at aapanel and select on Close.

After,

systemctl enable postfix && systemctl start postfix

Works like a charm!