[PLUG] Puzzling confirmation request

I just received a “confirmation request” that starts:

Your membership in the mailing list plug has been disabled due to
excessive bounces The last bounce received from you was dated
12-Jun-2019. You will not get any more messages from this list until
you re-enable your membership. You will receive 3 more reminders like
this before your membership in the list is deleted.

To re-enable your membership, you can simply respond to this message
(leaving the Subject: line intact), or visit the confirmation page at

<…snip…>

I then e-mailed plug-owner@lists.phillylinux.org to ask what this was about, as I have had my Gmail address ksbhaskar@gmail.com since 2004 and have never had complaints about mail delivery till now. Funnily enough, I have received e-mails from PLUG after this e-mail.

The response I got back when I e-mailed plug-owner was:

The response from the remote server was:

550 mail to plug-owner@lists.phillylinux.org not accepted here (#5.1.1)

Any idea what's going on? Thank you very much.

Regards

– Bhaskar

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