Mailer-daemon's reply for addresses of a deactivated mail domain says "loops back to myself"

Hello – and a great new year!

I deactivated a mail domain, not wanting to delete it, because I might want to reactivate it at some point. Testing the deactivation with an e-mail from another domain to a test address within the deactivated domain, I get a mailer-daemon reply with a failure notice containing this snippet:

Code:

Reporting-MTA: dns; [host]
X-Postfix-Queue-ID: 544A5141295
X-Postfix-Sender: rfc822; [me@my-domain]
Arrival-Date: Thu,  2 Jan 2020 15:45:05 +0100 (CET)

Final-Recipient: rfc822; test@[deact'd-domain]
Original-Recipient: rfc822;test@[deact'd-domain]
Action: failed
Status: 5.4.6
Diagnostic-Code: X-Postfix; mail for [deact'd-domain] loops back to
    myself

Is this what is designed to get back to senders of mail to a deactivated domain? Is there a way to change status and code to something different for this case?

This is ISPConfig 3.1 on Ubuntu 16.04 LTS set up according to "The Perfect Server – Ubuntu 16.04 (Xenial Xerus) with Apache, PHP, MySQL, PureFTPD, BIND, Postfix, Dovecot and ISPConfig 3.1".

Thanks & cheers,
Etc

 

If I send the email from another server (tried two different ones), it looks like the mail is being sent and I get nothing at all in reply (at least not within a few minutes), as if the mail would have been sent successfully. (I have removed the MX record for the domain now.)

 

More observations:

• The test mails didn't reach the test account (so this is as expected).
• I can still access the test account of the deactivated domain by IMAP.
• I can still send from the test account of the deactivated domain with SMTP.
• Both IMAP and SMTP even work when I check "Disable SMTP (sending)" and "Disable IMAP" for the specific mailbox.

 

Addendum: Now, after the MX record removal was propagated to the server's DNS, two earlier attempts at sending which at first seemed to be sent out ok were returned with a "Host or domain name not found" notice.

 

Pardon the possibly stupid question, but if I still have to deactivate every mailbox in it one by one, what does deactivating the mail domain actually do?

Back to the issue at hand, I did try every combination of activated/deactivated maildomain and activated/deactivated mailbox, including deactivated maildomain and deactivated mailbox ("enable receiving" off, "disable IMAP" on, "disable SMTP" on), but whatever I do, I can still access the mailbox via IMAP and use its credentials to send via SMTP.

 

Ok, another addition (success!): Somehow it seems the connection between Thunderbird and dovecot did survive several changes of settings to the maildomain and the mailbox in ISPConfig.

After closing and restarting both Thunderbird and manually restarting dovecot on the server the mailbox (maildomain deactivated + mailbox deactivated) is now inaccessible.