cannot add our mailing list as a subscriber

Our organization has a mailing list which has been used for years, and is still
used for issuing announcements. We also have a fairly new WordPress based blog that has
a separate follow-and-subscribe (Jetpack). I’d like to add our mailing list email as
a subscriber. The mailing list is moderated. There are a couple thousand subscribers
to the mailing list, but only 60 for the WordPress blog. There is no spamming going on
because the list and the blog are part of the same organization. I don’t want to import
2000 email addresses from the mailing list.

The problem is that we cannot seem to add the email address for posting to our mailing list.
For any of the 3 moderators of our mailing list (who get the “confirm subscription” email
after I add the mailing list email address),
the error that is returned is: Your subscription could not be activated. IT may have expired,
or the email address you subscribed with is not attached to your WordPress.com account.

Is there any way that I can add our mailing list post-to email address to the Jetpack
subscription box?

Thanks for any advice. I searched for plugins and options, and while I understand while manually
adding an email address (without confirmation) is generally not a good idea, it would really
help in this particular case.