Wildcard email addresses in MailMarshal

A few years back, there used to be a free service that enabled you to generate unique email addresses that would redirect to a single email account. This was great for web forms that required a valid email address. You would generate an email address for that particular website and disable it if they started spamming you.

Well, like all great "free" services, it eventually became "non-free", so that was the end of that. However, with a little ingenuity, it is possible to get MailMarshal to do something similar. Here's how:

Grammar

Firstly you need to identify the specific grammar of your email addresses, develop a secondary grammar for the wildcard addresses and then make sure there are no "collisions" in the grammar. For example, most organisations have their email addresses conforming (more or less) to the following grammar:

<first_name>.<last_name>@domain_name

That being the case, you can then define your wildcard grammer to be as follows:

<first_name>.<last_name>.<wildcard>@domain_name

Create Wildcard Group

The second step is to create a MailMarshal group called "Email Wildcard" and place the user names of everyone who will be using a wildcard, plus an entry with .* for every user as well. For example

joe.bloggs@example.com
joe.bloggs.*@example.com

It is possible to dispense with this step, however the group requirement gives you more control.

Rule 1

Some preliminary work is required here:

Firstly, create an external command called "Echo Old address to Threadno File". This is necessary because Mailmarshal can only work with non-SMTP header fields. You can work with the TO: field, but not the RCPT TO: field, which (unfortunately) is where the real stuff happens. So, we need to directly modify the text of the email outside of MailMarshal.

The external command will have the following properties:

Command Line: cmd
Parameters: /c echo {Recipient}>"{Install}\Address{Threadno}.txt
Tick the "single thread" and "execute once only" boxes. Leave the rest unchanged.

All this command will do is write the actual recipient email address to the file. Left like this, it will do nothing. We need to modify it later.

Next we need to write the rule to look for the email wildcard. This is done using the following Header-Match rule that needs to be manually defined as follows:

Match against: TO: CC: BCC:
Field parsing method: email addresses
Field search expression: (.+)\.(.+)\.(.+)@domain_name

For informational purposes, create a message stamp to indicate the email is from a wildcard source:

-----
Wildcard email address - {Recipient}

Next, a header rewrite rule needs to be created called "Address Change" as follows:

Match Against: X-AddressChange:
Field Parsing Method: Entire Line
Insert If Missing: Address Change

This will add the field X-AddressChange to the header indicating the email address has been changed and set us up for Rule 2. The complete Rule 1 will look as follows:

Standard Rule: Email Wildcard deletion - Rule 1
When a message arrives
Where message is incoming
Where addressed to 'Email Wildcard'
Where message contains one or more headers 'Email Wildcard'
Run the external command 'Echo Old address to Threadno File'
    And write log message(s) with 'Email Wildcard'
    And stamp message with 'Email Wildcard Stamp'
    And rewrite message headers using 'Address Change'
And pass message to the next rule for processing.

Rule 2

This rule is a lot simpler, it simply looks for the X-AddressChange field and then rewrites the email address to remove the wildcard.

The Header Match rule needs to be defined to look for "X-AddressChange" with the search expression '.+'

The Header Rewrite rule will be as follows:

Matching Fields: To:, Envelope Recipient:
Field Parsing Method: Email addresses
Field Search Expression: ^(.+)\.(.+)\.(.+)@(.+)
Substitute into field using expression: $1\.$2@$4
Enable Field Changes: (ticked) 

The final rule will be as follows:


Standard Rule: Email Wildcard deletion - Rule 2
When a message arrives
Where message is incoming
Where message contains one or more headers 'X-AddressChange Exists'
Rewrite message headers using 'Email Wilcard Deletion'
And pass message to the next rule for processing.

Rule 1 will stamp the message so you will know the original address used. If you start receiving spam from the source, add it to your blacklist of recipients. For example, suppose you sign up to a site using joe.bloggs.website1@example.com as your email address and you start being spammed by them. Add the address to your recipient blacklist and the spam will stop, however your regular email will still be delivered.

It is possible to add a "Rule 1.5" to add the recipient to the subject line - that way you can sort your emails by subject line. The rule would be very similar to Rule 2.

This is just one example of how you can push the boundaries of what MailMarshal is capable of by using external commands.