for file in **/**/*; do
  echo 'ENDOFMAILFILE' >> "$file"
done

input {
  file {
    type => "logmail"
    path => [ "/var/log/mail/**/*" ]
    start_position => "beginning"
    codec => multiline {
      pattern => "^ENDOFMAILFILE$"
      negate => "true"
      what => "previous"
    }
  }
}