Unlimited Mail Addresses

isotopp image Kristian Köhntopp -
January 20, 2021
a featured image

When people ask for my mail address, they usually get a personalized address from me. That is particularly true for all commercial email. So you don’t get to send mail to my main account, but to kris-yourbusiness@koehntopp.de , and that will end up going into INBOX.special.yourbusiness. At least until it leaks, receives spam or is otherwise burned. In which case I will short it out and route all incoming mail on that address to /dev/null. Here is how it is done.

Mailer setup

I run exim and in my exim.conf I have

  driver = redirect
  domains = +kris_domains
  local_part_suffix = +* : -*
  headers_remove = X-local-part-suffix
  headers_add = X-local-part-suffix: ${original_local_part}
  owners = kris
  data= ${lookup{$local_part@$domain}dbm*@{/home/kris/Exim/virtusertable.dbm}}

The important part here is the local_part_suffix = +* : -*. This enables amending the local part of a mail address with an extension. So if kris is a valid local user for delivery, kris-yourbusiness and kris+yourbusiness become valid addresses, too, and will be delivered to the local user kris.

I define local_part_suffix_optional, because unfortunately for legacy reasons the actual local user must remain reachable for legacy reasons.

I also define header_remove = X-local-part-suffix to remove this particular X-header from all incoming mail, and then add it back with X-local-part-suffix: ${original_local_part}. This will prevent external mail with a hand-crafted fake X-local-part-suffix header from coming through, and then preserve the original local part for sorting, in exactly this X-header line.

From this config we get the following behavior: kris-yourbusiness@koehntopp.de is being delivered to the local user kris, and with an X-local-part-suffix: kris-yourbusiness header line added to the mail. Note that we get kris-yourbusiness, and not yourbusiness in this header line.

Local delivery

Local delivery is controlled by the local user, kris. Kris happens to have defined a .forward such as

kris:~ $ cat .forward

This will invoke procmail for local delivery. procmail is a rule based local delivery agent with a horrible Syntax.

You can define variables, and later refer to them using $ as a prefix. Rules start with a :0, and consist of a Regex for header lines, plus a delivery action (usually a folder name for Maildir format, so make sure it ends in a slash).

The other important but obscure piece of information is that the exit code of procmail defines the delivery success, and controls the mailer behavior. So checking in the obvious place (/usr/include/sysexits.h, of course!) you learn:

kris:~ $ cat /usr/include/sysexits.h
#define EX_NOINPUT      66      /* cannot open input */
#define EX_NOUSER       67      /* addressee unknown */
#define EX_NOHOST       68      /* host name unknown */

We remember that the magical number 67 as an exit code makes exim bounce mail with user unknown.

The end result:

kris:~ $ cat .procmailrc


# Subrule to define a "user unknown" bounce, as an example.
# Subrule, because we also record the bounce in the bouncelog folder.
* ^X-local-part-suffix: kris-macheist

    :0 i

# Rule to route mail to the trash folder
* ^X-local-part-suffix: kris-bitly

# all mail to kris-yourbusiness goes to INBOX/.special.yourbusiness,
# which becomes the INBOX.special.yourbusiness folder.
# We use sed and tr, this could be more efficient. Basically, we clean
# up unwanted characters and transform to lower case.
* ^X-local-part-suffix:.*kris-\/[^@]+
$MAILDIR/.special.`echo $MATCH | sed -e 's!([^)]*)!!g' -e 's![^a-zA-Z0-9_-]!_!g' | tr A-Z a-z`/

# Filter through spamc
| /usr/bin/spamc

# If spam detected, deliver to different folders depending on score
* ^X-Spam-Level:.*\*\*\*\*\*\*\*\*\*\*

* ^X-Spam-Level:.*\*\*\*\*\*

Note how we detect the X-local-part-suffic before we run spam detection and sorting. This allows mail to subaddresses to always go through and bypass all spam filtering, so in exchange for having a personally identifiable mail address you get guarantee delivery. Conversely, if you abuse the privilege you are routed to $TRASH/.

Mail User Agent

I used to run Apple Mail.app with this setup, and now on Windows am using eM Client . eM Client is a commercial email client, and is available for Windows and Mac, and is easily the best email client I have ever used, with Mail.app being a close second for my workflow.

In my case, I define a smart folder search which will find all unread mail in all folders (except $list of exceptions), and show it in a single folder. I want the search to automatically discover newly generated folders automatically, and I need the folder name shown in the overview.

A smart folder, finding all unread mail anywhere, and showing it by folder. I made a few mails unread for demonstration.

That way, I can see if I get a mail claiming to be from YourBusiness: If it is not in the yourbusiness folder, I am deleting it without reading. Phishers will have to send phishing mail to the correct folder for your personalized business mail address in order to be even considered.

I also can $TRASH/ rule burned addresses rather quickly, and that keeps my inbox clean.