Fixing the "SMTP AUTH is required for message submission" error

[* Tong *]

Hi,

How to fix the "SMTP AUTH is required for message submission" error from
the client side? Is there any sendmail client-side configuration change
to *enforce* the SMTP AUTH command before all else?

The SMTP AUTH is required for message submission error seems to be a
common one, I found the same symptom here, http://error.bengtuo.com/
page/150171.html, which use PHP as the mail client, and here http://
forums.cpanel.net/f43/submission-message-port-587-without-smtp-
authentication-412522-p2.html#post1666792, which uses thunderbird as the
mail client. My question is specific for sendmail as the mail client.

This is what the error look like (and the reason):

Connecting to cpanelsvr.net. port 587 via relay...
220-cpanelsvr.net ESMTP Exim 4.80 #2 Sat, 21 Jun 2014 10:06:39
-0400
220-We do not authorize the use of this system to transport
unsolicited,
220 and/or bulk e-mail.
>>> EHLO myownbox.myplace.net
250-cpanelsvr.net Hello myownbox.myplace.net ...
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
>>> STARTTLS
220 TLS go ahead
>>> EHLO myownbox.myplace.net
250-cpanelsvr.net Hello myownbox.myplace.net ...
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250 HELP
>>> MAIL From:<mys...@hotmail.com> SIZE=29 AUTH=mys...@hotmail.com
250 OK
>>> RCPT To:<mys...@gmail.com>
>>> DATA
550 SMTP AUTH is required for message submission on port 587
mys...@hotmail.com... Deferred: Connection reset by cpanelsvr.net.

I.e., after STARTTLS sendmail *should have sent* the SMTP AUTH command
but it sent MAIL From and RCPT To instead.

What's interesting is that, I believe this is a server mis-configuration
issue, because as listed urls above, the symptom are all the same -- what
is working at other places will not work on a specific SMTP server, and I
believe such SMTP server is cPanel based.

For me, I configured our sendmail email server using smarthost to relay
to external mail relay servers. The same setting, basically pretty much
the same as this, https://www.dnsexit.com/support/mailrelay/sendmail.html,
works for two other of our upstream relay servers just fine, if I flip
the sendmail.mc smarthost entry and authinfo file to them. Here is the
log from one of our upstream relay server, which is working fine, showing
that the same sendmail.mc is working fine:

Connecting to our.upstream1.net via relay...
220 our.upstream1.net Microsoft ESMTP MAIL Service, Version:
6.0.3790.4675 ready at Tue, 17 Jun 2014 19:45:54 -0700
>>> EHLO myownbox.myplace.net
250-our.upstream1.net Hello [my.ip]
250-TURN
250-SIZE 41943040
250-ETRN
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-8bitmime
250-BINARYMIME
250-CHUNKING
250-VRFY
250-TLS
250-STARTTLS
250 OK
>>> STARTTLS
220 2.0.0 SMTP server ready
>>> EHLO myownbox.myplace.net
250-our.upstream1.net Hello [my.ip]
250-TURN
250-SIZE 41943040
250-ETRN
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-8bitmime
250-BINARYMIME
250-CHUNKING
250-VRFY
250-AUTH LOGIN PLAIN XOAUTH2
250 OK
>>> AUTH PLAIN c3VudG9...
235 2.7.0 Authentication succeeded
>>> MAIL From:<mys...@hotmail.com> SIZE=29 AUTH=mys...@hotmail.com
250 2.1.0 mys...@hotmail.com....Sender OK
>>> RCPT To:<antoni...@gmail.com>
>>> DATA
250 2.1.5 mys...@gmail.com
354 Start mail input; end with <CRLF>.<CRLF>
. . .

However, when I flipped them to the above erroneous server, I get the
above error. I'll stress again -- *nothing else is changed* except the
smart-host entry and authinfo.

Why does the "same" sendmail client-side configuration produce two
entirely different behaviour? Do you think it is because of the
"erroneous" server, or I missed some configuration entries at my (client)
side?

Thanks