Postfix
postfix(1) is an SMTP mail transfer agent.
Contents
Installation
Most Linux and BSD distributions offer a postfix package.
For systemd-capable systems, start and enable postfix.service.
For BSD distributions, try:
postfix start
Containers
postfix(1) is designed to be launched from userspace, rather than being a binary that can be invoked in the foreground. However, a new start-fg subcommand was added in version 3.3.
Consider the following Dockerfile as a template.
FROM alpine:latest RUN apk add --no-cache postfix EXPOSE 25 CMD ["postfix", "start-fg"]
To publish this service on an interface like 10.0.0.1, try:
sudo docker build --tag postfix . sudo docker run --detach --name my-postfix \ --restart=always \ --publish 10.0.0.1:25:25 \ postfix
Usage
Use a connection string like smtp+insecure+none://example.com:25.
Configuration
Before trying to configure Postfix, ensure that you understand the design of Postfix.
See also encryption and authentication.
Receiving Mail
Set myhostname and mydomain to the fully-qualified names. Set mydomains to the set of all 'trusted' networks. Set mydestination to the set of all domains that should be considered 'local'.
myhostname = www1.example.com mydomain = example.com mynetworks = 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 mydestination = $myhostname $mydomain www.$mydomain localhost localhost.localdomain
By default mail is only accepted...
from clients in trusted networks ($mynetworks)
- from clients that authenticated with SASL
for remote addresses matching $relay_domains
for local addresses found in $mydestination (defaulting to $myhostname, localhost.$mydomain, and localhost)
To adjust restrictions, try configuring smtpd_relay_restrictions or (the older and less-preferred method) smtpd_recipient_restrictions.
Local Delivery
For local addresses, the local part is extracted and casefolded to lowercase. This will be used to attempt delivery.
Mail is delivered to a user-specific folder under mail_spool_directory, i.e. /var/spool/mail/root. (Alternatively, mail can be delivered into users' home directories via home_mailbox.) The following manipulations are made to locally-delivered mail:
prepend a From SENDER DATETIME envelope header
prepend an X-Original-To: header
prepend an Delivered-To: header
prepend a Return-Path: header
prepend a > character to lines beginning with From
- append an empty line
Also, the mailbox is locked while delivery is in progress; if an error occurs, the mailbox is truncated to its original length. Delivery is executed with the permissions of the recipient.
Custom Delivery
A custom delivery command can be provided with mailbox_command_maps or mailbox_command.
In most cases, the command is executed with the recipient's permissions. If the recipient is root, a custom delivery command is executed with default_privs.
Qmail
For qmail-style mailboxes, the value of mail_spool_directory or home_mailbox must end in a forward slash (/).
home_mailbox = Maildir/
The following manipulations are made to locally-delivered qmail-style mail:
prepend a Delivered-To: header
prepend an X-Original-To: header
prepend a Return-Path: header
Forwarding
When attempting delivery, forward_path is scanned for a forward(5) file (i.e. ~/.forward). These looks like:
[email protected] # anything after # is ignored "|/path/to/examplemda"
Forwarded mail is sent as a new message with the Delivered-To: header, to prevent loops.
Note that the second line is only allowable if allow_mail_to_commands is set to:
allow_mail_to_commands = alias,forward,include
The default alias,forward disallows custom commands.
Routing
To route mail based on the recipient domain, try:
transport_maps = lmdb:/etc/postfix/transport
A transport(5) file (i.e. /etc/postfix/transport) looks like:
admin@localhost relay:[smtp.gmail.com]:587 service1.example.com lmtp:unix:/path/to/service.sock example.com lmtp:0.0.0.0:24 .example.com lmtp:0.0.0.0:24 localhost local .localdomain local * relay:[smtp.gmail.com]:587
The first part of each line is a pattern. The second part is an instruction:
a local instruction expands to the local_transport setting, which itself defaults to local:$myhostname
a lmtp instruction forwards mail to an LMTP server
a smtp instruction forwards mail to an SMTP server
a relay instruction causes mail to relayed
Bracketing an address prevents a MX record lookup; the A record alone is looked up and used naively. If even A record lookup should be skipped (i.e. for a name defined in the hosts file), additionally specify smtp_dns_support_level = disabled.
Domains prefixed with a dot (.) are a pattern for all subdomains. The example above captures localhost and *.localdomain for local delivery.
The asterisk (*) domain is a fallback route, used only if nothing else matches.
The matching happens in the hierarchical order shown above: by full address, then by full domain part, then by subdomain part, and finally the fallback.
Run postmap /etc/postfix/transport and a hashed file will be produced. If your postmap(1) does not use LMDB, replace the lmdb: with whatever algorithm was used.
Address Rewriting
See here.
Posting Mail
master(8) expects mail posted locally to use $myhostname as the sender's domain. To override this, set myorigin.
myorigin = $mydomain
Administration
Testing the service
Install mailx and send an empty email.
To test mail relay to external hosts, try:
mail -s 'Test Email' '[email protected]' </dev/null
Alternatively, try using telnet.
Reviewing the queue
Two useful administrative utilities exist for reviewing the mail queue: postqueue(1) and postcat(1).
To view the mail queue, try:
postqueue -p
This will display the queued messages, the senders and recipients, and a mail ID.
To force all queued mail to be sent now, run:
postqueue -f
To instead force a singular message to be send now, run:
postqueue -i MAILID
To instead inspect a message in the queue, try:
postcat -vq MAILID