if (/^To: *auto@domain.com$/)
   to this_mail | python ~/script.py command

$ tail -n 1 /etc/aliases
fakeuser: "| /tmp/fakescript"

$ cat /tmp/fakescript
#!/bin/bash
cat - >> /tmp/log

$ chmod +x /tmp/fakescript

$ echo test | mail -s 'subject 1' fakeuser
$ echo test | mail -s 'subject 2' fakeuser

$ grep '^Subject:' /tmp/log
Subject: subject 1
Subject: subject 2