錯誤 postfix/sendmail: fatal: open /etc/postfix/main.cf: Permission denied
如果這個錯誤存在，大部分情況就無法寄信。
修正
SELINUX
# setsebool httpd_can_sendmail on -P
錯誤 cannot update mailbox /var/mail/root for user root. error writing message: File too large
postfix/local: D86CF60AA4FC: to=<user@example.com>, orig_to=<root>, relay=local, delay=0.23, delays=0.13/0/0/0.09, dsn=5.2.2, status=bounced (cannot update mailbox /var/mail/root for user root. error writing message: File too large)
主要是信箱容量到達設定上限
查看上限
mailbox_size_limit = 51200000
修改上限
預設 51.2 MB，改大一點，修改 /etc/postfix/main.cf
加上一行，改為512MB
mailbox_size_limit = 512000000
重啟postfix 生效
清空root信箱, root 也可以改成使用者信箱
錯誤，google拒絕收信 Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked
postfix/smtp: 4A2E960A9CDB: to=<user@gmail.com>, orig_to=<service@xxx>, relay=gmail-smtp-in.l.google.com[142.251.8.26]:25, delay=1.9, delays=0.8/0/0.32/0.77, dsn=5.7.1, status=bounced (host gmail-smtp-in.l.google.com[142.251.8.26] said: 550-5.7.1 Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. Please visit 550-5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError 550 5.7.1 for more information. 195si8062422pfz.220 - gsmtp (in reply to end of DATA command))
解決
進到網頁
https://support.google.com/mail/?p=UnsolicitedMessageError
https://support.google.com/mail/answer/9981691?visit_id=637823184872027838-3968779762&rd=1
https://postmaster.google.com/managedomains?pli=1
把你的網域加入，例如我是從 @n.sfs.tw 寄出，則新增此網域並在dns加入txt的記錄後進行驗證即可。
還不行的話DNS 加入SPF記錄也許能解決問題。
參考資料
[1] https://ma.ttias.be/postfix-cannot-update-mailbox-varmail-user-file-large/