gnus-button-prefer-mid-or-mail

gnus-button-prefer-mid-or-mail is a variable defined in `gnus-art.el'.
Its value is

gnus-button-mid-or-mail-heuristic

Documentation:
What to do when the button on a string as "foo123@bar.invalid" is pushed.
Strings like this can be either a message ID or a mail address. If it is one
of the symbols `mid' or `mail', Gnus will always assume that the string is a
message ID or a mail address, respectively. If this variable is set to the
symbol `ask', always query the user what do do. If it is a function, this
function will be called with the string as its only argument. The function
must return `mid', `mail', `invalid' or `ask'.

You can customize this variable.

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