Try to grant user "vmail@localhost" also.
Jul 8 04:34:03 mail postfix/postscreen[18321]: CONNECT from [127.0.0.1]:41127 to [127.0.0.1]:25
Jul 8 04:34:03 mail postfix/postscreen[18321]: WHITELISTED [127.0.0.1]:41127
Jul 8 04:34:03 mail postfix/smtpd[18322]: connect from mail.demo.com[127.0.0.1]
Jul 8 04:34:03 mail postfix/smtpd[18322]: NOQUEUE: reject: RCPT from mail.demo.com[127.0.0.1]: 451 4.3.0 <me@demo.com>: Temporary lookup failure; from=<me@demo.com> to=<muratdemir3475@gmail.com> proto=ESMTP helo=<mail.demo.com>
Jul 8 04:34:03 mail postfix/smtpd[18322]: disconnect from mail.demo.com[127.0.0.1]
Jul 8 04:34:38 mail postfix/postscreen[18321]: CONNECT from [127.0.0.1]:41135 to [127.0.0.1]:25
Jul 8 04:34:38 mail postfix/postscreen[18321]: WHITELISTED [127.0.0.1]:41135
Jul 8 04:34:38 mail postfix/smtpd[18322]: connect from mail.demo.com[127.0.0.1]
Jul 8 04:34:38 mail postfix/smtpd[18322]: NOQUEUE: reject: RCPT from mail.demo.com[127.0.0.1]: 451 4.3.0 <me@demo.com>: Temporary lookup failure; from=<me@demo.com> to=<muratdemir3475@gmail.com> proto=ESMTP helo=<mail.demo.com>
Jul 8 04:34:38 mail postfix/smtpd[18322]: disconnect from mail.demo.com[127.0.0.1]
Jul 8 04:34:47 mail postfix/pickup[16818]: C6835CA1316: uid=0 from=<root>
Jul 8 04:34:47 mail postfix/cleanup[17960]: warning: proxy:mysql:/etc/postfix/mysql/sender_bcc_maps_user.cf lookup error for "root@mail.demo.com"
Jul 8 04:34:47 mail postfix/cleanup[17960]: warning: C6835CA1316: sender_bcc_maps map lookup problem -- message not accepted, try again later
Jul 8 04:34:47 mail postfix/pickup[16818]: warning: maildrop/B0D8CCC283A: error writing C6835CA1316: queue file write error