Bug#828801: Legitimate mail can contain long lines

[Gedalya]

Package: exim4-config
Version: 4.87-3

Ref: #797919

I've noticed the recent addition to acl_check_data in conf.d/acl/40_exim4-config_check_data:

  deny    message    = maximum allowed line length is 998 octets, \
                       got $max_received_linelength
          condition  = ${if > {$max_received_linelength}{998}}

I decided to try it out on a server getting some real traffic. It turns out the overwhelming majority of messages hit by this are spam, but not all.

Many legitimate and important messages coming from legitimate sources apparently have long lines. These tend to be non-human-originated messages such as order confirmations and alerts of various kinds.

These are coming from some major email sources such as sendgrid, mailgun, our favorite: GoDaddy, and others, and some from the sender's own network, such as Verizon.

I tested manually against gmail, and they let a long line (1150 octets) right through. Interestingly, Postfix 2.11.3-1 from jessie accepted the long line from the client but broke it into two [0] *after* DKIM as added by opendkim, so the signature was broken. Exim versions prior to 4.87 of course let it through. So in conclusion, we can't assume no legitimate mail will contain long lines, so this is probably not a good default configuration for a mail exchanger receiving mail from all over the wild.

[0] http://www.postfix.org/postconf.5.html#smtp_line_length_limit