Thursday, July 8, 2010

Relaying - How does IndiMail handle relaying securely

A SMTP server is responsible for accepting mails from a sender and processing it for delivery to one or more recipients. In most situations, for domains which are under your administrative control (native addresses), the SMTP server should accept mails without authentication. However, when a mail is submitted for delivery to domains which are not under your administrative control, you should accept mails only after it satisfies security considerations like having the sender authenticate itself. This is to prevent abuse of external domains using your SMTP server. A SMTP server which accepts mails for external domains without any authentication is called an open relay. The act of accepting mails for external domains for delivery is called relaying.

The default configuration of IndiMail configures the SMTP as a closed system. Hence to be able to send mails to external domains, you need to setup mechanisms for relaying.

There are many methods. Choose any of the below after studying them. I prefer 3 or 4 for security reasons.
  1. Have Sender's IP addresses in tcp.smtp file
  2. Use control file relayclients for IP addresses of clients allowed to relay mail through this host.
  3. Configure IndiMail to use MySQL relay table (good security). This is implemented on POP3/IMAP before SMTP
  4. Use authenticated SMTP (good security)
  5. For allowing relay to specific domains use control file relaydomains
  6. For allowing specific users (native addresses) use control file relaymailfrom
  • NOTE: you should use 1 & 2 only if if the host having the sender's IP is under your control and you have good security policies for the host (however what is a good security can be very subjective)
Using tcp.smtp

Your startup script for the qmail smtp server must use the tcpserver -x file option similar to this startup line.

env - PATH="/var/indimail/bin" tcpserver -H -R -x /var/indimail/etc/tcp.smtp.cdb \
-c 20 -u 555 -g 555 0 smtp /var/indimail/bin/qmail-smtpd 2>&1

IndiMail uses -x option to tcpserver and hence you need not bother about the above line. You however need to edit /var/indimail/etc/tcp.smtp and put in lines for all static IP's that you will always want to relay access to.


The above lines will cause RELAYCLIENT environment variable to be set for localhost and all machines on the 10.1.1 class and hence allow to relay through. Remember that any user on hosts on 10.1.1 class will be able to relay mails. You many not want this. The line having 127.0.0. will allow any client on the IndiMail host to use SMTP and relay mails.

If you add any IP to tcp.smtp, you have to rebuild a cdb database tcp.smtp.cdb. You can run the following command

% sudo /var/indimail/bin/qmailctl cdb

NOTE: Remember that you are exposed to unrestricted relaying from any of the IP addresses listed in tcp.smtp

Using control file relayclients

IP addresses of clients allowed to relay mail  through  this  host.   Each address should be followed by a colon and an (optional) string that should be appended to each incoming recipient address, just as  with  the  RELAYCLIENT environment variable.  Nearly always, the optional string should be null. The filename can be overriden by  the  environment  variable  RELAYCLIENTS.

Addresses in relayclients may be wildcarded (2nd line in the example below):

Using MySQL relay table

Run the command /var/indimail/bin/clearopensmtp in the cron every 30 Minutes

By default every time who uses IndiMail's  POP3 or IMAP service and authenticates, the following happens:
  1. On successful authentication, IMAP/POP3 daemon inserts entry into relay table, inserting email, IP address and timestamp.
  2. If CHECKRELAY environment variable is enabled, SMTP checks the relay table for a entry within minutes specified by the RELAY_CLEAR_MINUTES environment variable. If the entry is there, RELAYCLIENT environment variable is set, which allows relaying. At this point, the SMTP server will allow that IP to relay for 60 Mins (default)
clearopensmtp will clear all IP which have not authenticated in the past RELAY_CLEAR_MINUTESclearopensmtp should be enabled in cron to run every 30 minutes.

Set up Authenticated SMTP

IndiMail also provides you authenticated SMTP providing AUTH PLAIN, AUTH LOGIN and AUTH CRAM-MD5 methods. Whenever a user successfully authenticates through SMTP, the RELAYCLIENT environment variable gets set. qmail-smtpd uses the RELAYCLIENT environment variable to allow relaying.

Most of the email clients like thunderbird, evolution, outlook, outlook express have options to use authenticated SMTP.

For a tutorial on authenticated SMTP, you can refer to

Using control file relaydomains

Host  and  domain  names  allowed  to  relay mail through this host.  Each  
address should be followed by a colon and an (optional) string that should 
be  appended  to  each incoming recipient address, just as with the RELAY\
CLIENT environment variable.  Nearly always, the optional string should be 
Addresses in relaydomains may be wildcarded:

Using control file relaymailfrom

envelope  senders (MAIL FROM) listed in this file will be allowed to relay independently of the RELAYCLIENT environment variable. Entries in relaymailfrom can be E-Mail addresses, or just the domain (with the @ sign).

Unlike relaydomains native addresses should be entered.  A line in  relay mailfrom  may be of the form @host, meaning every address at host.  relaymailfrom can also be in cdb format. If relaymailfrom.cdb  is  present,  it will be searched first.


If you use the control file /var/indimail/control/relaymailfrom, you should really know what you are doing. Any mail from having a domain component of the address matching any domain in this file, relaying will be allowed without any authentication. You can most probably use this only if you have a closed SMTP server to which access from outside is not possible.

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