mm-w3m-safe-url-regexp

mm-w3m-safe-url-regexp is a variable defined in `mm-decode.el'.
Its value is

"\\`cid:"

Documentation:
Regexp matching URLs which are considered to be safe.
Some HTML mails might contain a nasty trick used by spammers, using
the tag which is far more evil than the [Click Here!] button.
It is most likely intended to check whether the ominous spam mail has
reached your eyes or not, in which case the spammer knows for sure
that your email address is valid. It is done by embedding an
identifier string into a URL that you might automatically retrieve
when displaying the image. The default value is "\\`cid:" which only
matches parts embedded to the Multipart/Related type MIME contents and
Gnus will never connect to the spammer's site arbitrarily. You may
set this variable to nil if you consider all urls to be safe.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.