I created a linux user via webmin in CentOS.
username was webAdmin ( which includes a capital ).
i couldnt able to send e-mails to the user.
here is the reason
http://www.cyberciti.biz/faq/linux-usernames-with-capital-letters-not-working/
i changed webAdmin to webadmin, now everything is OK.
here they talk about another solution using the “u” flag in sendmail.cf for sendmail :
http://archives.neohapsis.com/archives/postfix/2000-11/0017.html
i didnt test it, i had better keep them all non-capitals anymore. 
