If you set mail_always and then try to run "sudo su user" the su command will fail:

[thiago@hermes thiago]$ alias w=whoami
[thiago@hermes thiago]$ w
thiago
[thiago@hermes thiago]$ sudo su teste
[thiago@hermes thiago]$ stty: standard input: Input/output error
[teste@hermes thiago]$ w
thiago
[thiago@hermes thiago]$ [teste@hermes thiago]$
[thiago@hermes thiago]$ exit

[thiago@hermes thiago]$

I alias'ed w to whoami because after the error any key you type is accepted as a command, thus when I tried to type whoami it issued the command 'w', then 'a', then etc.

My sudoers:
Defaults        syslog=auth, mail_always, insults
User_Alias      OPERADOR =      user1,\
                                user2,\
                                thiago
Cmnd_Alias      SU = /bin/su, /usr/bin/passwd
root    ALL=(ALL) ALL
OPERADOR        hermes = /bin/su [!-]*,\
                         !/bin/su *root*,\
                         !/bin/su *OPERADOR*
OPERADOR        hermes = /usr/bin/passwd [!-]*,\
                         !/usr/bin/passwd *root*,\
                         !/usr/bin/passwd *OPERADOR*

By the way, *OPERADOR* doesn't work, I was only testing :-).

I still received the command log by mail as expected, though.