A typical IE toolbar has a little "envelope" picture, usually next to the "print" icon and on the same section of the bar as the "Word" icon.
You can also click on the "File" and hover on "Send" to select send page, send link, or put a shortcut to the page on your desktop.
A third entry to the same functions is on "Tools" "Mail and News" where you can select "send a link" or "send page."
A suspected/possible reason why sending a page might fail is simple file size. This thread, prior to this post with only four posts, receives as a 37 KB email. Most threads aren't going to be large enough to cause a problem, but 50 short posts like those here would be close to 1 MB, and a thread with longer and/or more posts could easily reach the 2 MB limit imposed by many email services.
Smaller individual messages could also fail delivery if the recipient has a "stuffed full" mailbox and there isn't room to receive a new one of the size you send. We have several contacts who have to be telephoned to remind them to empty their in-box almost every time we send them an email with more than a few words.
One simple way to check out what's happening would perhaps be to put yourself on as a CC: or BCC: recipient, so you'll know whether the problem is with the sending or the getting of the message(?)
A second possibility, if you're not putting a clear "sent by" ident and an appropriate subject on the message is that some of the messages are going into junk mail at the addressees' end. The default subject line, for a mudcat thread, is just "Emailing: thread.htm" which really looks sort of like junk.