reject the empty envelope-sender, <>

方法1：
main.cf，添加以下設定

smtpd_restriction_classes = submission_sender_checks

submission_sender_checks =
   check_sender_access hash:/etc/postfix/reject_null_sender

master.cf，添加以下設定
#  existing submission entry
submission ... smtpd
  ... other options ...
#  add sender restrictions
  -o smtpd_sender_restrictions=$submission_sender_checks

新增reject_null_sender，並增加以下內容
# reject_null_sender
<>  REJECT null sender not allowed

sudo postmap reject_null_sender 

sudo service postfix restart

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方法2：
main.cf添加以下設定

smtpd_reject_unlisted_sender = yes