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[Postfix]: check_sender_access: How to catch envelope FROM with brackets?

Placeholder

10/30/23, 3:36 PM

Overall goal:

Block specific emails with have "a name" included in their FROM envelope. Example: John <john@example.com>

Current situation:

I have successfully configured the main.cf of Postfix to utilize check_sender_access in smtpd_recipient_restrictions.

I have created my own sender_access file, used postmap on it, so it generates sender_access.db, which is used then by main.cf.

My sender_access file looks like this:

john@example.com REJECT
support@example.com REJECT

This configuration does works as intended and rejects the email when somebody sends an inbound email with "clear" FROM envelope (Example: FROM: support@example.com):

This configuration does not work and reject the email when the sender has set a "name" which appears in the FROM envelope (Example: FROM: John <john@example.com> or FROM: Support <support@example.com>):

The configuration "does not catch" the email address either when the "name" is the same as the email address (some default behavior in some MTA /mail transfer agents/):

What I have tried:

I have tried a sender_access file with the following formatting:

john@example.com REJECT
John <john@example.com> REJECT
"John <john@example.com>" REJECT
John REJECT
<john@example.com> REJECT
"john@example.com" REJECT
john@ REJECT

None of these have practically worked. The only thing which works is if I reject the entire domain:

example.com REJECT

My question:

I have been searching for two days now, tried many articles and finally decided to ask here. Does Postfix support blocking of senders with FROM in the format John <john@example.com>?

Do I need to try some regex (interestingly, I didn't found any - one would think this is something commonsense?).

P.S: Perhaps something which always "catches" the email address inside the brackets in bold:

John <john@example.com>

Thank you!

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postfix

glts

10/30/23, 5:39 PM

Your terminology is confused. You are not actually talking about the ‘envelope from’. The ‘envelope from’ would be the [MAIL FROM command](https://www.rfc-editor.org/rfc/rfc5321#section-4.1.1.2) which does not allow a name at all, *only* a mailbox in angle brackets. The envelope is usually not visible in a web interface.

glts

10/31/23, 9:14 AM

Perhaps a more helpful comment, if you are asking about the `From:` header then have a look at the manual page *header_checks*(5) (`man 5 header_checks`), it might just be what you’re looking for. Or ask on the Postfix mailing list.

Placeholder

11/3/23, 4:23 PM

^--- Your suggestions are all wrong. But thanks anyway, E for Effort. :)

Score:0

Server

Placeholder

11/3/23, 3:38 PM

Solution:

I solved this problem by using regular expressions (regexp) checks.

main.cf

smtpd_recipient_restrictions = check_sender_access regexp:/etc/postfix/sender_access

sender_access

# Exact match: john@example.com
/^john@example\.com$/ REJECT

# Advanced match: John <john@example.com>
/john@example\.com/ REJECT

Note: Please be aware if you use only /john@example.com/ without terminating with \ the . character, then it will match all characters in that position except linebreaks (Example: john@example$.com , john@example!.com, etc.)

Testing

You can test your sender_access via with the command:

postmap -q "John <john@example.com" regexp:sender_access

Do not forget to use the following commands in order to make sure your changes take effect:

postmap sender_access
service postfix restart

Additional notes:

1. If you add a new rule within the sender_access file, doing only a postmap sender_access seems to be enough.

2. Sometimes even though the email appears in the frontend to come from John <john@example.com it is actually emailed from an address like <12323323487210921@mail.example.com>. This is especially through if the sender uses a service such as Amazon SES.

In this case you must block the actual domain (Example in sender_access: /^john@example\.com$/ REJECT). You can see the real address in your Postfix logs:

vim /var/log/mail.log

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