Using a shell account as a manual sendmail relay

So the situation is like this: An email I attempted to send got rejected by the recipient’s mail server because my ISP (Netvision) has a poor spam reputation. And it so happens that I have a shell account (with root, possibly) on a server with an excellent reputation. So how do I use this advantage?

On my Thunderbird oldie, save the message with “Save As…” from the “Sent” folder into an .eml file.  Or from “Unsent Mail” folder, if it’s a fresh message which I haven’t even tried to send the normal way (using the “Send Later” feature).

Copy this .eml file to the server with good mail reputation.

On that server, go

$ sendmail -v -t < test.eml
"eli@picky.server.com" <eli@picky.server.com>... Connecting to [127.0.0.1] via relay...
220 theserver.org ESMTP Sendmail 8.14.4/8.14.4; Sat, 18 Jun 2016 11:05:26 +0300
>>> EHLO theserver.org
250-theserver.org Hello localhost.localdomain [127.0.0.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-DELIVERBY
250 HELP
>>> MAIL From:<eli@theserver.org> SIZE=864
250 2.1.0 <eli@theserver.org>... Sender ok
>>> RCPT To:<eli@picky.server.com>
>>> DATA
250 2.1.5 <eli@picky.server.com>... Recipient ok
354 Enter mail, end with "." on a line by itself
>>> .
250 2.0.0 u5I85QQq030607 Message accepted for delivery
"eli@picky.server.com" <eli@picky.server.com>... Sent (u5I85QQq030607 Message accepted for delivery)
Closing connection to [127.0.0.1]
>>> QUIT
221 2.0.0 theserver.org closing connection

The -v flag causes all the verbose output, and the -t flag makes sendmail read the headers. If there’s a Bcc: header, it’s removed before sending.

IMPORTANT: If the processing involves several connections, they will be shown one after the other. In particular, the first leg might be to the local mail server, and only then the relaying out. In that case, the EHLO of the first connection is set by the sendmail executable we’re running, not the sendmail server.

Note that the “MAIL From:” (envelope sender) is the actual user on the Linux machine (user name@the machine’s domain name). The -f flag can be used to used to change this:

# sendmail -v -f 'eli@billauer.co.il' -t < test.eml

To be sure it went fine, look in /var/log/maillog. A successful transmission leaves an entry like this:

Jun 18 11:06:17 theserver sendmail[30611]: u5I85QQq030607: to=<eli@picky.server.com>, ctladdr=<eli@theserver.org> (500/123), delay=00:00:51, xdelay=00:00:48, mailer=esmtp, pri=120985, relay=picky.server.com. [108.86.85.180], dsn=2.0.0, stat=Sent (OK id=1bEBGd-0007kL-DB)

Note the mail ID, which was given by sendmail (marked in red). Finding all related log messages is done simply with e.g. (as root)

# grep u5I85QQq030607 /var/log/maillog
Jun 18 11:05:26 theserver sendmail[30607]: u5I85QQq030607: from=<eli@theserver.org>, size=985,, nrcpts=1, msgid=<57650054.90002@picky.server.com>, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Jun 18 11:05:29 theserver sendmail[30604]: u5I85NZV030604: to="eli@picky.server.com" <eli@picky.server.com>, ctladdr=eli (500/123), delay=00:00:06, xdelay=00:00:03, mailer=relay, pri=30864, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (u5I85QQq030607 Message accepted for delivery)
Jun 18 11:06:17 theserver sendmail[30611]: u5I85QQq030607: to=<eli@picky.server.com>, ctladdr=<eli@theserver.org> (500/123), delay=00:00:51, xdelay=00:00:48, mailer=esmtp, pri=120985, relay=picky.server.com. [208.76.85.180], dsn=2.0.0, stat=Sent (OK id=1bEBGd-0007kL-DB)

(the successful finale is the last message)