The notifications center (see T1000) and DevCentral uses incoming mail facility from Mailgun.

Mailgun has moved incoming mails facility as a premium feature, requiring 35$ / month.

Plan is to build our own incoming mail facility with transactional e-mail.

A previous attempt to solve this problem was Lamson, built on the top of Django by Zed Shaw around 2010-2012, used in a service called LibreList, but unmaintained afterwards.

It's probably a better bet to write an application getting a mail from the MTA (MTA like Postfix can directly pass mail to a command), sorting it and able to send it to an app or a broker:

Mail --SMTP--> Postfix --stdout--> {small app to write} --HTTP--> DevCentral/Notifications Center
                                                        \-AMQP--> Broker