There are several ways to have a linux box email you when someone logs in to it. Most of these use a script in either the local profile files (for individual users) or the system-wide profile (and/or in sshrc). Another nice way is to use the pam authentication system to do the job. A setup is given at:
http://blog.stalkr.net/2010/11/login-notifications-pamexec-scripting.html
Assuming Debian here.
Make sure the system is set up to talk to an email server and has some way of sending emails from the command line. The script here assumes the mailx package is installed.
Quick test:
echo "test" | mail -s "test" user@example.com
Create a script somewhere sensible (e.g. /usr/local/bin) and make it executable.
#!/bin/sh [ "$PAM_TYPE" = "open_session" ] || exit 0 { echo "User: $PAM_USER" echo "Ruser: $PAM_RUSER" echo "Rhost: $PAM_RHOST" echo "Service: $PAM_SERVICE" echo "TTY: $PAM_TTY" echo "Date: `date`" echo "Server: `uname -a`" } | mail -s "`hostname -s` $PAM_SERVICE login: $PAM_USER" user@example.com
replacing user@example.com with the required email address. Note that you could send the email to root or another local account if you have that aliased to an external address.
edit /etc/pam.d/common-session to add at the end:
session optional pam_exec.so /usr/local/bin/login_notify
and test it by logging in with an ssh session (terminal or winscp etc.)
Sudo uses common-session-noninteractive - not sure whether it would be best to put this in /etc/pam.d/sudo as well…