Messages not going to List

I’ve set up a Mailing List in WPMG.
I’m using MailGun as my SMTP gateway
I have tested the SMTP server using WP_Mail and can send a message out using the Troubleshooting function. In the Mailgun logs, I see the message received and delivered.

What I don’t see, is the second part of the equation – where the message is sent to my mailing list recipients.

To test this further, I completed the following steps:
I have emailed from an address that is on the list and active.
I see the email come into my imap mailbox
I see WPMG receive one queued message in the troubleshooting screen
The message disappears from the IMAP mailbox
The count for queued messages returns to 0.
No message is received by any of the addresses on that list.
Mailgun does not show a connection attempt for the messages that have been received.

I’m sure I’ve missed somewhere. I really want to buy the premium version of this but won’t if the base functionality doesn’t work.
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