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#>’, ‘<#H#>’(*5), ‘<#l#>’, ‘<#L#>’(*5) and ‘<#n#>’ with the list host name, host name, local name, unmodified local name, and the current message number, respectively. For the digest of the list ‘mailinglist@example.org’ 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.