[exim] [Transport error]: message has lines too long for tra…

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Author: Andrea Biscuola
Date:  
To: exim-users
CC: riccardo.brunetti
Subject: [exim] [Transport error]: message has lines too long for transport
Hi all

We are an italian hosting provider (https://host.it) and we use exim to relay e-mails from
our hosting servers.
Exim is installed and configured through the directadmin control panel
(https://www.directadmin.com), so the main configuration is managed using the
directadmin custombuild subsystem.

One week ago, we upgraded to exim 4.95 and suddenly, some customers (using microsoft
outlook, nonetheless), started to experience the following error for *some* of their
e-mails:

       message has lines too long for transport
    Reporting-MTA: dns; web017.shared.host.it


I received some examples of such e-mails from our customers service, and it appear that
the problem is with some badly formatted headers.

Unfortunately, we can't throw those customers out of the window :-) so we are searching
how to expand the line limits for the transports.
However, I'm struggling to understand, from the documentation, what the correct solution
is. From what I was able to understand, we should modify the remote_smtp and
remote_smtp_forward_transport sections to solve the problem, with the message_linelength_limit
parameter set to something like "4096".

Consider also, that the main exim configuration, is handled directly by directadmin and
it's generated from it's internal "templates". In general, we can safely change a configuration
file called /etc/exim.variables.conf.custom to put a series of overrides to the default
configuration.

Do we need to modify the directadmin templates or can we use one of the "custom" files
in /etc to achieve the same result?

Thank you and best regards.

Andrea