Problem with upper/downer letters in emailadresses

[Christian Neugebauer]

Hello, 
  
I am using Postfix and Cyrus. 
If I send a local mail to user at host, everything works fine, 
but if I send to User at host, the mail is not delivered and I get the  
message "Mailbox does not exist". 
 
Is there any possibility to let postfix rewrite the address to lower letters, 
or to let Cyrus ignore upper letters? 
 
Thanks for any help 
Christian Neugebauer