Headers in DIR/headeradd are added to posts before they are sent to subscribers. They are also added to the archived version of the message.
ezmlm will substitute the tags <#h#>, <#l#>, and <#n#> with the list host name, local name, and the current message number, respectively. For the digest of the list ‘email@example.com’ the local name is ‘mailinglist-digest’. The current message number for digests is the number of the first message in the digest. You can use this to e.g. add a sequence number header by putting ‘X-Seq: <#n#>’ into DIR/headeradd.