You aren't using LDAP integration with Postfix are you? If so, that may perhaps be the issue? (FWIW, it's not possible to use Postfix LDAP integration within an unpriviledged container & v16.x won't work as a priviledged container without a bit of work).

Otherwise, you are right. It should at least respond and there should be confirmation of mail sent in /var/mail/root. Although I'll need to set one up with SendInBlue before I can give you any further guidance. I'll respond ASAP, but unfortunately, it may not be until early next week...

Off the top of my head though, I don't think Gmail supports SAML username/password authentication anymore (IIRC it only support oAuth2 token authentication). And if your SendinBlue account is new, then you need to enable "Transactional Emails" within your account (sorry we need to document that more clearly). And that sometimes takes a little while before it works.

I've finally circled back to test this myself to see if I could reproduce your issue. I launched an unpriveledged LXC container of our v16.1 Observium appliance on Proxmox.

After initial install, within Confconsole, I configured it to use my (personal) SendinBlue account (I created a new SMTP key within the SMTP tab of the SMTP & API section).

Then I tested as per your notes. Here's what I got:

root@jed-test-observium ~# echo "Subject: sendmail test" | sendmail -v jeremy@turnkeylinux.org
Mail Delivery Status Report will be mailed to .
root@jed-test-observium ~# cat /var/mail/root 
From MAILER-DAEMON  Mon Jun  7 05:51:01 2021
Return-Path: 
X-Original-To: root@observium
Delivered-To: root@observium
Received: by observium (Postfix)
	id C852A142D5; Mon,  7 Jun 2021 05:51:01 +0000 (UTC)
Date: Mon,  7 Jun 2021 05:51:01 +0000 (UTC)
From: MAILER-DAEMON@observium (Mail Delivery System)
Subject: Mail Delivery Status Report
To: root@observium
Auto-Submitted: auto-replied
MIME-Version: 1.0
Content-Type: multipart/report; report-type=delivery-status;
	boundary="AE304141B4.1623045061/observium"
Content-Transfer-Encoding: 8bit
Message-Id: 

This is a MIME-encapsulated message.

--AE304141B4.1623045061/observium
Content-Description: Notification
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit

This is the mail system at host observium.

Enclosed is the mail delivery report that you requested.

                   The mail system

: delivery via
    smtp-relay.sendinblue.com[34.93.126.74]:587: 250 Message queued as
    

--AE304141B4.1623045061/observium
Content-Description: Delivery report
Content-Type: message/delivery-status

Reporting-MTA: dns; observium
X-Postfix-Queue-ID: AE304141B4
X-Postfix-Sender: rfc822; root@observium
Arrival-Date: Mon,  7 Jun 2021 05:50:58 +0000 (UTC)

Final-Recipient: rfc822; jeremy@turnkeylinux.org
Original-Recipient: rfc822;jeremy@turnkeylinux.org
Action: relayed
Status: 2.0.0
Remote-MTA: dns; smtp-relay.sendinblue.com
Diagnostic-Code: smtp; 250 Message queued as
    

--AE304141B4.1623045061/observium
Content-Description: Message Headers
Content-Type: text/rfc822-headers
Content-Transfer-Encoding: 8bit

Return-Path: 
Received: by observium (Postfix, from userid 0)
	id AE304141B4; Mon,  7 Jun 2021 05:50:58 +0000 (UTC)
Subject: sendmail test
Message-Id: 
Date: Mon,  7 Jun 2021 05:50:58 +0000 (UTC)
From: root 

--AE304141B4.1623045061/observium--

And not only did the test appear to work correctly within my server; the test email arrived in my inbox too!

So I'm a bit unsure why my experience was so radically different to yours?!

Could you please confirm which version your appliance is? If you're not sure, please post the output of:

turnkey-version

The only other thing that occurs to me is that perhaps there have been some config and/or package changes that have broken postfix in your appliance? Perhaps it's worth migrating to a new appliance? Let me know if this is an option for you and I can give you some adive on the easy way to do that.