[exim] Wildcard CN verify error

I'm getting an SSL verify error for messages delivered through my ESP :

    SSL verify error: certificate name mismatch: DN="/C=US/ST=Texas/L=San
Antonio/O=MAILGUN TECHNOLOGIES, INC/CN=*.mailgun.org" H="smtp.mailgun.com"

Based on Jeremy's comment in this old bug report, the wildcard CN isn't an
issue.

    https://bugs.exim.org/show_bug.cgi?id=2011

Is there a particular configuration option I need to set to get it to
verify?

My transport:

    smtp_mailgun:
      driver = smtp
      hosts_require_auth = <; $host_address
      hosts_require_tls = <; $host_address
      tls_certificate = /etc/letsencrypt/live/fullchain.pem
      tls_privatekey = /etc/letsencrypt/live/privkey.pem

Thanks!
Lance