This text will be prepended to subject lines of messages
posted to the list, to distinguish mailing list messages in in
mailbox summaries. Brevity is premium here, it's ok to shorten
long mailing list names to something more concise, as long as it
still identifies the mailing list.
You can also add a sequencial number by %%d substitution
directive. eg.; [listname %%d] -> [listname 123]
(listname %%05d) -> (listname 00123)