More info on refused E-mail

To avoid being bombarded which junk E-mail, mail is refused on the following grounds:

Your ISP's computer is trying to conceal its identity.
You are sending mail with a non existing sender address.
You are trying to send a file which is too large.
Your ISP's computer has been blacklisted.
You are using spam specific strings.
Software bugs and configuration errors.

Details

1

Internet computers have both a name and a number. If the name and number do not match or if the name cannot be established at all, a rogue site is assumed.

2

This could be a configuration error but usually means that someone is trying to conceal his / her identity.

3

To avoid `mail bombing' and reduce data costs, the size of incoming E-mails is limited.

4

Spam friendly ISP's are blacklisted. If your mail is refused as a result of blacklisting, your mail + a reference to the blacklist is sent back to you. In this case you should send an E-mail to postmaster@Your_Isp.
If this problem persists, the best thing to do is to change provider.

5

These include bold statements about economic activities and the extensive use of capitals.

6

Most common mistakes and bugs;

Hostname
See 1.
Helo
Before the mail is being send, the sending computer tells it's name. If this name is wrong, the remote system appears to be infected with a virus or worm. The most common mistakes are
- A helo ending '.lan', '.local' or some other non existent top level domain.
- Using a hostname that belongs to an other computer.
Mail from
See 2.
Rcpt to
The recipient doesn't exist or there is a spelling error in the recipient's email address.
Content
Apart from [5], there may be software bugs. Most common bugs are;
- Non ASCII chars in headers. The message headers should only contain the following characters;
```
     ! " # $ % & ' ( ) * + , - . / 0 1 2 3 4 5 6 7 8 9 : ; < = > ?
   @ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _
   ` a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~
   
```
  Other glyphs, such as 'ë', 'ï', or 'ö' should be 'coded' Eg;
```
   To: =?UTF-8?Q?Andr=C3=A9?= <Andre@example.com>
   
```
  For André.
  The message body may contain non ASCII glyphs, as long as it is clear which character set is being used. Eg;
```
   Content-Type: TEXT/PLAIN; charset=UTF-8; format=flowed
   
```
- To: <undisclosed-recipients:;>
  'undisclosed-recipients: ;' is a correct recipient address. So is 'undisclosed-recipients: ; <>'. '<undisclosed-recipients:;>' is not.

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