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Email Address and Advanced SMTP Filtering Control

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Updated information at: Classic Linux.

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An advanced user interface to SMTP filtering can be used by going to and following the link entitled “Email Address and Advanced SMTP Filtering Control”.

To enable or disable SMTP filtering, check or uncheck the ENABLE box. Checked will give you less spam and unchecked will give you more spam. Also from among A, B, C, …, X select one or more.

The ENABLE checkbox acts like a master OFF switch. If it is left cleared (i.e., unchecked), then SMTP filtering will not be active, regardless of whether any of A, B, C, …, X are checked.

A, B, and C will very rarely, if ever, block legitimate email. C is slightly more aggressive than B but not unduly so.

  • A: Greylisting. See smtp_filtering_to_keep_out_spam and references therein.
  • B: Black lists that list (a) individual IP addresses of misconfigured open proxies and virus-infected Microsoft machines, (b) netblocks of locations with dynamic IP addresses, and (c) individual IP addresses or small netblocks of the most egregious spam sources.
  • C: Other conservative black lists that list both individual IP addresses and larger network blocks that appear to be dedicated to sending spam.

Enabling D, E, and X should eliminate almost all spam, while allowing an occasional false hit. If you enable any of D, E, or X, then B and C will also become active.

  • D: Strict checking of Internet standards, e.g., DNS, hostname, SMTP HELO.
  • E: Aggressive black lists that include sending locations believed to be spam-friendly or spam-suspect.
  • X: Extremely aggressive black lists and new or experimental techniques.

To find out more details about what each of A, B, C, …, X does, look for a link entitled “Details about A, B, C, …, X” near the bottom of the SMTP Filtering user interface.

You don't have to stick to any one combination of SMTP filtering options. Simply add a number of email addresses (see Removing Email Addresses) and set the SMTP filtering for each as you prefer. For example, email addresses that you give to close friends or family can be left only mildly filtered, while email addresses that you publish on a web page may need to be heavily filtered.

Finding Out Which Mail was Blocked

You can find out which mail arriving for you was blocked by SMTP filtering with the help of the scan.logs command.

Motivated and Non-Motivated Senders

Regardless of how aggressively an email address is filtered, a motivated sender should always be able to get through to you. When SMTP filtering blocks a message, the blocked message is rejected with a valid SMTP code. The sender will get back an error indication from his mail system. So the sender will know that his email did not get through. If he really wants to reach you, he will find some way. Contrast this with the more conventional approach in which spam gets refiled into a spam folder. If the intended recipient doesn't check the spam folder and notice a misfiled non-spam message, the non-spam message is silently lost. The sender thinks it reached, but the recipient never notices it. SMTP filtering does not let mail get silently lost this way–either the recipient gets it, or the sender knows that the recipient did not get it.

But non-motivated senders will sometimes not reach you if SMTP filtering is enabled. A non-motivated sender is one who discards or ignores sending errors or, knowing that an error occurred, makes no noticeable effort to solve the problem that caused his email to you to fail. Ask non-motivated senders to send you email at an email address that is completely non-SMTP-filtered.

Mailing Lists

Many operators of mailing lists are non-motivated senders. Subscribe to such mailing lists using a hyphenated suffix email address (see Email Hyphenated Suffixes) and keep SMTP filtering disabled for that email address. You may need to subscribe to the mailing list twice: once with your “normal” email address so you can conveniently send mail to the list, and again with the hyphenated suffix email address at which you receive mail from the list. Set your subscription options so the list sends no mail to your “normal” email address, but still recognizes that email address as a subscriber to the list.

However, one result of the above strategy might be that many people on the mailing list will have your “normal” email address in their address-books. Sooner or later, this email address will leak out into the global Internet, and you will begin getting (more) spam at it. A less spam-prone option is to use the same hyphenated email address to send mail to the list. If you can, use a mail program (e.g., Alpine) that can automatically set your from email address based on the address to which you send mail. If in the future you begin getting too much spam at the hyphenated email address, simply forward that address to “fail” and re-subscribe to the mailing list with a new hyphenated email address.

Country-Specific Blocking

You can block all mail from certain countries of your choice by following the link from within the Advanced interface to the Baroque interface. Then select the countries to block using two-character codes below. Countries are identified by the IP address of the remote host that connects to our servers to send mail.

  • ar: Argentina
  • au: Australia
  • br: Brazil
  • ca: Canada
  • cn: China
  • es: Spain
  • fr: France
  • hk: Hong Kong
  • in: India
  • jp: Japan
  • kr: Korea (South)
  • ru: Russia
  • sg: Singapore
  • tw: Taiwan
  • gb: Great Britain

Old Filtering

The Baroque interface will also let you select an option “OLD”. This enables SMTP filtering at a level similar to the way it used to be in March 2006, before the new options became available. The OLD filtering can be activated only if options B and C are both unchecked (to avoid duplicate processing, because B and C include everything that OLD does).


Warning: The descriptions above are approximate. Black lists and techniques in use may change without notice. IP address-to-country mappings are believed to be close to 100% accurate but are not guaranteed to be so.

classic/email_address_and_advanced_smtp_filtering_control.txt · Last modified: 2021/01/30 04:53 by admin