Your mail server domain is misconfigured.
If postfix has reject_unknown_sender_domain enabled, all mails got rejected:
NOQUEUE: reject: RCPT from mail-s38.mailgun.us[184.173.153.38]: 450 4.1.8 bounce+ff81b3.23346e-invoiceplan=example.org@mailgun.invoiceplane.com: Sender address rejected: Domain not found; from=bounce+ff81b3.23346e-invoiceplan=example.org@mailgun.invoiceplane.com to=invoiceplan@example.org proto=ESMTP helo=<mail-s38.mailgun.us>