I've an iMac with OS X 10.4.11. Sudden email problem: every received email has, after the header, 1 - 2 pages of technical data (e.g., Dkim-Signature, Domainkey-Signature, etc.) I don't know why this now happening, and I don't know how to get rid of it.

I have an iMac using OS X 10.4.11. Beginning several days ago, all incoming emails have, after the usual email header and before the message appears, one to two pages of technical data (e.g., Dkim-Signature, Domainkey-Signature, Content-Transfer-Encoding, etc.). This is similar and more complex that the info one receives when an undeliverable email bounces back. I don't know where this stuff comes from, or how it came, and I don't know how to get rid of it. Any ideas? I'm something of a newbie to computers (um, what's a Tag?).