cut length and Change subject

I want to forward all incoming emails via sms to a gsm-telefon and keep
a copy on my local account.
To send the mail to the telefon, i need to change the subject to a
password (string , 4 chars). The max length of the mails to the telefon
is 160 chars.
how will this work?
Sounds easy? Not for me. Sorry.
I hope that someone can send me a receipt
Thanx Dieter