A first time user of
memberlist.php?mode=email
has little idea of what the finished message will look like to the recipient.

The only hint is "Please note that the topic information is already included in the message."

That indeed translates to a whopping:

Hello Dan J,

This e-mail was sent from "..." by 積丹尼 who thought you may be
interested in the following topic:

大家好唷...

You can find it at:

http://www.example.com/forum/viewtopic.php?f=5&t=45...

A message from 積丹尼 may also be included below. Please note that this
message has not been seen or approved by the board administrators. If you
wish to complain about having received this e-mail please contact the board
administrator at example@... Please quote the message headers
when contacting this address.

Solution. Show the entire thing that is about to be sent in a readonly textarea or something. With a read-write textarea below it for their additional message.