Does mailx send mail using an SMTP relay or does it directly connect to the target SMTP server?

Question

Suppose i send a mail using the following the following command:

mailx person@x.com

then does mailx first try to find out the SMTP server of my ISP for relaying the mail or does it connect directly. Does it depend on whether my PC has a public IP address or it is behind a NAT. How do I check the settings of mailx on my PC? How can I verify this using tcpdump?

Answer 1

Normally, mail and derivatives (and almost any other Unix MUA) use the traditional sendmail interface - /usr/bin/sendmail, provided by almost all MTAs (postfix, exim, sendmail, courier). This is the traditional Unix way.

Once a message is submitted through sendmail, your MTA handles message transmission. Depending on configuration, it may either connect directly to the destination MTA (if on a server), or relay mail through another host (also called a smarthost) - the latter is more common on personal computers, where relaying through your Gmail or ISP/work email account is essential.

(Some MTAs such as esmtp or nullmailer are built specifically for home users and always use a relayhost. These don't support receiving mail and are a lot lighter on resources.)

mailx → /usr/bin/sendmail → local MTA → recipient MTA → recipient inbox

mailx → /usr/bin/sendmail → local MTA → Gmail or ISP/work servers → recipient MTA → recipient inbox

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Other programs (mostly the user-friendly graphical clients such as Thunderbird or Outlook) connect directly to a relay SMTP server (again, usually Gmail or ISP/work SMTP server), which transmits the message further.

SMTP support is present in heirloom-mailx, but not in the traditional bsd-mailx.

app → Gmail or ISP/work servers → recipient MTA → recipient inbox

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The third method - directly to recipient's server - is almost never used, and no MUA supports it. On personal computers, using it would cause your message to get rejected (a lot of spam is sent from infected home user IP addresses).

Answer 2

mailx can use SMTP. It's configure file is ~/.mailrc

One example is mailx using Gmail's SMTP.

The configure can even be in one command:

mailx -v -s "$EMAIL_SUBJECT" \
-S smtp-use-starttls \
-S ssl-verify=ignore \
-S smtp-auth=login \
-S smtp=smtp://smtp.gmail.com:587 \
-S from="$FROM_EMAIL_ADDRESS($FRIENDLY_NAME)" \
-S smtp-auth-user=$FROM_EMAIL_ADDRESS \
-S smtp-auth-password=$EMAIL_ACCOUNT_PASSWORD \
-S ssl-verify=ignore \
-S nss-config-dir=~/.mozilla/firefox/xxxxxxxx.default/ \
$TO_EMAIL_ADDRESS

If a normal SMTP server is used, it is much easier:

mailx -v -s "$EMAIL_SUBJECT" \
-S smtp=smtp://smtp.server.com
-S from="$FROM_EMAIL_ADDRESS($FRIENDLY_NAME)" \
$TO_EMAIL_ADDRESS

Answer 3

From the mailx(1) man page, DESCRIPTION section, String Options subsection:

   smtp   Normally, mailx invokes sendmail(8) directly to  transfer
          messages.  If the smtp variable is set, a SMTP connection
          to the server specified by the value of this variable  is
          used  instead.

Answer 4

echo this is my message | mail -sSubject -Ssmtp=smtp.east.cox.net youraddress@yourhost.com