+(defcustom message-valid-fqdn-regexp
+ (concat "[a-z0-9][-.a-z0-9]+\\." ;; [hostname.subdomain.]domain.
+ ;; valid TLDs:
+ "\\([a-z][a-z]" ;; two letter country TDLs
+ "\\|biz\\|com\\|edu\\|gov\\|int\\|mil\\|net\\|org"
+ "\\|aero\\|coop\\|info\\|name\\|museum"
+ "\\|arpa\\|pro\\|uucp\\|bitnet\\|bofh" ;; old style?
+ "\\)")
+ "Regular expression that matches a valid FQDN."
+ ;; see also: gnus-button-valid-fqdn-regexp
+ :group 'message-headers
+ :type 'regexp)
+