The built-in goto-address-mode uses heuristics to identify URLs and
email addresses in the current buffer. It then applies a face to them
to change their style. Some packages, such as ‘notmuch’, use this
minor-mode automatically.
The faces are not declared with defface, meaning that it is better
that the theme does not modify them. The user is thus encouraged to
consider including (or equivalent) this in their setup:
(setq goto-address-url-face 'link
goto-address-url-mouse-face 'highlight
goto-address-mail-face 'link
goto-address-mail-mouse-face 'highlight)
My personal preference is to set goto-address-mail-face to nil, as
it otherwise adds too much visual noise to the buffer (email addresses
stand out more, due to the use of the uncommon ‘@’ character but also
because they are often enclosed in angled brackets).