[Dovecot] turn Usernames in Lowercase

Hi,

I'm just curious how to force usernames to be modified to lowercase letters if one mixes uper and lower case letters to one salad? Cause my SpamAssassin gets confuesd and treats different spellings of Usernames as different Users - which is uncool. I've heard about something you can add in your dovecot.config to enforce lowercase letters only.

Thanks