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

linuxmailxnat;smtp

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?

Best Answer

Traditionally, Unix mail and derivatives (and many other Unix tools) use the /usr/bin/sendmail interface, provided by almost all mail transfer agents (MTAs – postfix, exim, courier, and of course sendmail).

That is, the mail program doesn't speak any network protocol – it feeds the message to sendmail via stdin, and lets it handle actual delivery. (This goes back to the days when some mail used SMTP, some used UUCP, some used BITNET...)

Once a message is queued through sendmail, the MTA handles actual message transmission, whether through SMTP or something else. Depending on configuration, it may either connect directly to the destination MTA, or relay mail through another host (also called a smarthost).

Direct connection is more common on servers; relay via smarthost is more common on personal computers on home connections – relaying through your Gmail or ISP/work email account is essential to avoid the blanket "dynamic IP" anti-spam filters.

(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 queue → [SMTP] → recipient MTA → recipient inbox
mailx → [/usr/bin/sendmail] → local MTA queue → [SMTP] → Gmail or ISP/work servers → [SMTP] → recipient MTA → recipient inbox

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

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

app → [SMTP] → Gmail or ISP/work servers → [SMTP] → recipient MTA → recipient inbox

The third method – connecting 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).

app → [SMTP] → recipient MTA → caught by the spam filter
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