Send Mail Safely with Postfix's sendmail Interface

Type sendmail on a modern Debian or Ubuntu box and you are almost never running the original Sendmail: you are running Postfix's compatibility shim. On this machine the commands come from Postfix 3.8.6-1ubuntu0.1, and Postfix accepts only a useful subset of the old interface, quietly ignoring some recognised Sendmail options.

• Time: about fifteen minutes.
• Needs: a shell account and a working Postfix installation.
• Risk: the submission example sends real mail, so replace every placeholder and use a destination you control. Queue inspection is read-only. Editing aliases and rebuilding their database changes local delivery and needs elevated privileges.

1. Confirm which compatibility commands are installed

Start with ordinary, read-only checks. The three names are related interfaces: sendmail submits mail, mailq lists queued mail, and newaliases rebuilds alias databases.

$ command -v sendmail mailq newaliases
/usr/sbin/sendmail
/usr/bin/mailq
/usr/bin/newaliases
$ dpkg-query -W -f='${Package} ${Version}\n' postfix
postfix 3.8.6-1ubuntu0.1

On this installation all three names resolve to the Postfix sendmail compatibility binary. That matters when you compare examples from Sendmail documentation.

2. Inspect the queue before testing delivery

Run the read-only queue listing. mailq and sendmail -bp select the same mode.

$ mailq
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
... queue entries appear here ...
-- 5 Kbytes in 1 Request.

Your output will be host-specific. Each entry includes a queue ID, size, arrival time, sender and recipients still awaiting delivery.

• Trailing * marks an active item.
• Trailing ! marks a held item.
• Trailing # marks mail forced to expire.
• A reason line below an entry explains a failed delivery attempt.

Do not treat a non-empty queue as proof that Postfix is broken. A remote DNS failure, a refused connection or a destination server delaying mail can all leave a message queued. Record the queue ID before investigating.

$ sendmail -bp
$ printf 'status: %s\n' "$?"
status: 0

3. Submit one deliberately addressed test message

Prepare the message as standard headers followed by a blank line and a body. Use -- before the recipient so an address beginning with a hyphen cannot be interpreted as another sendmail option.

$ sendmail -v -- [email protected] <<'EOF'
From: [email protected]
To: [email protected]
Subject: Postfix sendmail test

This is a delivery test from the Postfix sendmail interface.
EOF

Replace both addresses with real values. The -v option requests a report about the first delivery attempt, but mail delivery still happens in the background. A successful command only means Postfix accepted the message for processing. It does not prove that the remote mailbox accepted it.

By default, sendmail reads standard input until end of file or a line containing only a full stop. The here-document above supplies end of file explicitly. The -- is also a security boundary when any recipient value is supplied by another user or program. Do not run Postfix commands as root on behalf of untrusted users without filtering their arguments.

4. Verify acceptance without assuming delivery

Immediately list the queue again:

$ mailq
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
... the test message may appear here ...

A message may disappear quickly if delivery succeeds, or remain while Postfix retries it. If you need a one-off routing check that does not collect or deliver a message, use -bv with a recipient:

$ sendmail -bv -- [email protected]
... a delivery verification report is sent ...

The report checks address rewriting and routing rather than delivering the message. It may itself be sent as email, so inspect the command's standard error and the local mail logs if the result is not clear. A queued message's reason is more useful than repeatedly flushing the queue.

The -q option attempts delivery of all queued mail. Treat it as an operational action, not a harmless retry button: the manual warns that frequent flushing of undeliverable mail harms delivery performance for other messages. Use it only when you understand the backlog and the cause of the delay.

5. Change aliases with a recoverable plan

Postfix reads the alias databases named by its alias_database setting. Check the current setting before editing anything:

$ postconf -h alias_database default_database_type
hash:/etc/aliases
hash

Here the source file is /etc/aliases and the database type is hash.

1. Back up the source.

   $ sudo cp -p /etc/aliases /etc/aliases.backup
   $ sudoedit /etc/aliases

2. Rebuild the database. Add or edit only the alias you intend to change, then rebuild:

   $ sudo newaliases
   $ printf 'status: %s\n' "$?"
   status: 0

3. Verify it took.

   $ sudo ls -l /etc/aliases /etc/aliases.db
   $ postmap -q ALIAS_NAME hash:/etc/aliases
   TARGET_VALUE

newaliases runs the Postfix alias-database update using the configured files and database type. It can take about a minute for an update to become visible; the sendmail manual recommends postfix reload when you need to eliminate that delay.

$ sudo cp -p /etc/aliases.backup /etc/aliases
$ sudo newaliases
$ sudo postfix reload

Do not use newaliases as a general queue repair command: it only updates alias databases. Likewise, editing /etc/aliases.db directly is the wrong recovery path, because the source file is the durable configuration.

6. Keep the compatibility boundary clear

Common options have Postfix-specific meanings:

• -f or -r sets the envelope sender used for delivery errors.
• -t adds recipients found in message headers to any given on the command line.
• -B 7BIT or -B 8BITMIME describes the body type, but binary or non-ASCII content still needs suitable MIME encoding.
• -I and -bi are aliases for the same alias-database initialisation mode as newaliases.
• -bd starts Postfix, and -bl starts it with loopback-only intent when the configuration sets inet_interfaces = loopback.

The last three are service operations, not routine message-submission flags. Use the postfix service controls and a maintenance plan when changing daemon state.

Postfix writes problems to standard error and to the system mail logs. When a command appears to succeed but delivery does not, check the queue entry and logs before changing configuration. Avoid adding sudo reflexively: queue viewing and submission normally use the invoking user's permissions, while alias and service administration are the cases that need privilege.