Rejecting false "mail from" addresses

Rejecting false "mail from" addresses

   KB 20383        Last updated on 2015-07-20  

(2 votes)

By default any connection made to ZCS postfix and declares "mail from: local sender" (even if it is not) - the connection/email is accepted for local delivery. This wiki provides steps to block such connections. Once following is configured, postfix will accept "mail from: local sender" only if the connection made from a hosts in "mynetworks" OR the sender is sasl authenticated.

Modify "smtpd_sender_restrictions". We are adding a check before allowing a normal smtp connection. Allowing hosts in mynetwork, then allowing sasl authenticated too. Then a check for local domain address. If its true - the connection will be rejected.

Zimbra Collaboration 8.x

For Zimbra Collaboration 8.x, open the file /opt/zimbra/conf/zmconfigd/ and add this line into the enf of the file: Add this at the end of the file:

 check_sender_access lmdb:/opt/zimbra/conf/domainrestrict

Zimbra Collaboration 8.0.x

For Zimbra Collaboration 8.0.x, open the file /opt/zimbra/conf/zmconfigd/ and add this line into the middle of the file, prior to the lines: Add this:

 check_sender_access hash:/opt/zimbra/conf/domainrestrict

Should looks like:

check_sender_access hash:/opt/zimbra/conf/domainrestrict
%%contains VAR:zimbraServiceEnabled antivirus^ check_sender_access regexp:/opt/zimbra/postfix/conf/

Zimbra Collaboration 7.x

For Zimbra Collaboration 7.x you should follow the next steps:

 su - zimbra
 zmlocalconfig -e postfix_smtpd_sender_restrictions="reject_unknown_sender_domain, permit_mynetworks, permit_sasl_authenticated, check_sender_access hash:/opt/zimbra/conf/domainrestrict, permit"

Remaining steps are same for all versions

  • Create the file "/opt/zimbra/conf/domainrestrict" and add your domain(s) to it.   REJECT   REJECT

You can also put some friendly/non-friendly message. Something like this.   REJECT   You're not me! REJECT   You're not me!
  • Create the hash database of "/opt/zimbra/conf/domainrestrict". Run as 'zimbra' user.
 postmap  /opt/zimbra/conf/domainrestrict
  • Restart zmmtactl.
 zmmtactl stop
 zmmtactl start


Make following connection from a non-local host which is not part of mynetworks.

 telnet ZCS_server_address 25
 mail from:
 rcpt to:

You should get following error at the rcpt command.

 554 5.7.1 <>: Sender address rejected: You're not me!

Special case of empty 'mail from' address

Emails can still be sent if the 'mail from:' address is blank, but the 'from' address is specified in the body of the email. This is expected behaviour, and is required by RFC 3464:

The From field of the message header of the DSN SHOULD contain the address of a human who
is responsible for maintaining the mail system at the Reporting MTA site (e.g., Postmaster), so that
a reply to the DSN will reach that person.
Whenever an SMTP transaction is used to send a DSN, the MAIL FROM command MUST use a
NULL return address, i.e., "MAIL FROM:<>".

If you want to Enforce a match between FROM addres and SASL username, use the next Wiki for Zimbra Collaboration 8.5 and above:

Verified Against: Zimbra Collaboration 8.0, 7.0 Date Created: 04/16/2014
Article ID: Date Modified: 2015-07-20

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