Greetings,

Encountered some issues during the night with three FreeBSD servers I 
have running. They all generate automatic system emails during the night 
and these emails are all sent to one mailing list on one of the servers. 
The mail system on the receiving side is using the latests courier from 
FreeBSD ports which is courier 0.65.3 and the mail list is handled by 
couriermlm.

The email last night seemed to contain something odd as the email 
generated on the receiving server together with the one from one of the 
other servers was rejected by couriermlm like this:
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: This message looks like 
an administrative request.  The address
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: <mailto:[REMOVED]> is NOT
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: for administrative 
requests.  This is the posting address for
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: the mailing list 
itself.  For additional help, write to
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: <mailto:[REMOVED]>.  To 
reach
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: the mailing list 
administrator, write to
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>: <mailto:[REMOVED]>.
Jun  6 03:29:24 [REMOVED] courierlocal: 
id=[REMOVED],from=<[REMOVED]>,addr=<[REMOVED]>,status: failure

I'm guessing the above means that a bounce is generated and in our 
setup, that bounce would be sent to the very same mailing list that 
generated this error and if the bounce contains the original message 
that would be an infinite loop.

I can see in the maillog that this error occurs three times in a row for 
the email generated on the local machine. The first has the correct from 
address while the second has a empty from address and the third has #@[] 
set as from address. All this happens within one second and I see no 
trace of it after that and the mail queue is empty.

The second server (running qmail) encountered the same error but just 
once and then got a backscatter error like this:
Jun  6 03:46:48 [REMOVED] courieresmtp: 
id=[REMOVED],from=<>,addr=<[REMOVED]>: 250 Backscatter bounce dropped.
Jun  6 03:46:48 [REMOVED] courieresmtp: 
id=[REMOVED],from=<>,addr=<[REMOVED]>,success: delivered: backscatter 
bounce dropped
Jun  6 03:46:48 [REMOVED] courieresmtp: 
id=[REMOVED],from=<>,addr=<[REMOVED]>,status: success

On the sender side (qmail) it looks as if the message was delivered 
successfully.

The third server (running courier) however encountered the problem with 
556 Address unavailable, like this:
Jun  6 04:05:29 [REMOVED] courieresmtpd: 
error,relay=[REMOVED],from=<[REMOVED]>,to=<[REMOVED]>: 556 Address 
unavailable.
Jun  6 04:05:29 [REMOVED] courieresmtpd: error,relay=[REMOVED],msg="502 
ESMTP command error",cmd: DATA

Just like the local errors on the reciever server I see three attempts 
here too and I see all three attempts on both sender and receiver side. 
First attempt that gets a 556 has the correct from address, second 
attempt has an empty from address and also gets a 556, the third has the 
#@[] from address and that one gets a 517 Syntax error and thats the end 
of it.

I've been looking at logs on both sides and I can find no reason for the 
556 error. I get why the local delivery and the delivery from the second 
server fail as it could enter an infinite loop but the 556 I cannot 
understand.

We've tried to reproduce the error without success. Mail to the list 
worked before this error and has for months and now works again without 
any change to configuration.

Also, the empty and the #@[] from addresses I see, is that intended or 
have we missed something in our configuration?

Regards,
Andréas

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