Below is the full text to include/mail.h from NetHack 3.4.3. To link to a particular line, write [[mail.h#line123]], for example.

1.    /*	SCCS Id: @(#)mail.h	3.4	1991/10/11	*/
2.    /* NetHack may be freely redistributed.  See license for details. */

3.    
4.    /* used by ckmailstatus() to pass information to the mail-daemon in newmail() */
5.    
6.    #ifndef MAIL_H
7.    #define MAIL_H
8.    
9.    #define MSG_OTHER 0	/* catch-all; none of the below... */
10.   #define MSG_MAIL  1	/* unimportant, uninteresting mail message */
11.   #define MSG_CALL  2	/* annoying phone/talk/chat-type interruption */
12.   
13.   struct mail_info {
14.   	int	 message_typ;		/* MSG_foo value */
15.   	const char *display_txt;	/* text for daemon to verbalize */
16.   	const char *object_nam;		/* text to tag object with */
17.   	const char *response_cmd;	/* command to eventually execute */
18.   };
19.   
20.   #endif /* MAIL_H */