Issue when allowing reply-to posts and email to create topics

Is it supported to enable reply by email enabled and also create a custom incoming email address when using the simple mail-receiver? It seems my install will only work with one or other enabled:

  1. Set custom incoming email address for a category
  2. Inbound emails correctly create new topics
  3. Set reply by email enabled true
  4. Inbound email to category then bounce.

The error in the bounced email is " 550 5.1.1 Requested action not taken: mailbox unavailable"

I think the relevant section of ./launcher logs mail-receiver is:

<23>Mar 15 23:18:46 receive-mail[250]: Recipient:
<22>Mar 15 23:18:47 postfix/pipe[249]: 694343BAE8D: to=<>, relay=discourse, delay=0.73, delays=0.33/0/0/0.4, dsn=2.0.0, status=sent (delivered via discourse service)
<22>Mar 15 23:18:47 postfix/qmgr[80]: 694343BAE8D: removed

In the log is the custom email address for the category.

A possibly-related question: Is there a default inbound email address for a category (based on the category slug?), or does a custom one have to be set?