Bug#987981: exim4: misleading bounce and error message "message is too big (transport limit = 1)"

[Frank Heckenbach]

Package: exim4
Version: 4.92-8+deb10u5
Severity: normal

When I tried to send a mail, exim sent me this bounce:

  Subject: Mail delivery failed: returning message to sender

  This message was created automatically by mail delivery software.

  A message that you sent could not be delivered to one or more of its
  recipients. This is a permanent error. The following address(es) failed:

    [...]

  --
  Content-type: message/delivery-status

  Reporting-MTA: dns; mars

  Action: failed
  Final-Recipient: rfc822;[...]
  Status: 5.0.0

which contains no indication of what's wrong. (AFAICG, 5.0.0
actually means "Message Not Delivered: Unknown issue").

In the log it says:

  message is too big (transport limit = 1)

which is also misleading (the message was not very large, and what
does "transport limit = 1" even mean, 1 what?).

I had to google this message to finally find some other reports
containing it which hinted at the 998 character line length limit
per RFC.

This was indeed the problem with my mail, but nothing in what exim
told me gave any indication of that.

Improving the bounce seems more important, since it's the sender,
not the admin, who can usually fix the problem. But improving the
log message, too, would also help.

-- System Information:
Debian Release: 10.9
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-debug'), (500, 'proposed-updates-debug'), (500, 'proposed-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 5.6.0-0.bpo.2-amd64 (SMP w/24 CPU cores)
Locale: LANG=de_DE, LC_CTYPE=de_DE (charmap=ISO-8859-1), LANGUAGE= (charmap=ISO-8859-1)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages exim4 depends on:
ii  debconf [debconf-2.0]  1.5.71
ii  exim4-base             4.92-8+deb10u5
ii  exim4-daemon-light     4.92-8+deb10u5