This is the Frequently Given Answer to such behaviour. You can find this same exhortation in a vast number of general guides to discussion forum netiquette (such as The Usenet Posting Rules, Walt Howe's Guidelines For Posting In Public, The Prodigy Guide For Posting Messages, Marlene M. Maheu's article on Usenet Newsgroups, The "24 Hour Support" guide to "Netequitte" [sic] and Online Harrassment's Guide to Newsgroups) and in most of the rules for privately-owned discussion fora (such as The Macromedia Forums Posting Guide, The SBC Yahoo! Netiquette Rules, and The 4Rants Forum Rules, to pick but three examples).
When joining a discussion forum - any discussion forum, be it a Fidonet echo, a Usenet newsgroup, or a mailing list - always endeavour to read the back messages before you begin posting. Ideally, read all of the back messages. But at the very least try to read several months' worth.
When joining a Fidonet echo, read the back messages that are available from your uplink. If necessary, send AreaFix (or whatever the local equivalent is) the commands to set your lastread pointers in that echo back a few weeks and to schedule a rescan.
When joining a Usenet newsgroup, read the back messages in that
newsgroup that have not yet scrolled off the news server. If there
aren't more than a couple of day's worth because your news server
expires messages quickly, then (a) look for a news server with a greater
capacity and longer expiry interval, and (b) make use of
DejaNewsGoogle.
When joining a mailing list, make use of any facility that the mailing list manager robot provides for sending you copies of previous messages posted to the list, and read whatever archives of the mailing list are available. If there are no "back issue" facilities and no archives, lurk for a week or so.