To block anyone (local (mail/sendmail command) system users and SMTP users) from sending to an email address you cannot rely on
smtpd_recipient_restrictions. You need to place the restriction into the
qmgr phase. For this I've found that
transport_maps works well.
transport_maps = pcre:/etc/postfix/transport_maps
Maybe there is a better solution but this one appears to work for all delivery types. FYI, that regex supports
email@example.com assuming a
+ delimiter. It prevents To, CC and BCC.
Also make sure your postfix has pcre support enabled. On Debian based (Ubuntu, etc) operating systems that is provided by the postfix-pcre package.