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+## Thư bị chặn
+
+550-5.7.1 LIEBHERR spam recognition! Please add U-mgdA2BMa4iL in subject AND body and re-send your email after 15 minutes. 
+550-5.7.1 CYREN IP reputation determined a medium risk associated with the sender address 171.244.9.134. 
+550 5.7.1 MAIL FROM domain (return.prmailmx.com) and Header-From domain (ckda.vn) are not equal.
+
+- Lý do: Mail gửi từ domain ckda.vn có header-from (return.prmailmx.com) không trùng tên domain.
+- Gửi lại thư: add U-mgdA2BMa4iL vào subject,body và gửi lại thư sau 15p
+- Khắc phục: Xác thực các bản ghi của domain trên postman.
+
+## Server nhận yêu cầu có bản ghi MX
+451 <pmrp.dau-thau.com> does not have an MX record
+
+## Server postman thiếu ip reverse DNS
+550-5.5.1 Your server IP address [171.244.9.123] is missing a reverse DNS 550-entry. See http://spamauditor.org/best-practices/check-ip-reverse-dns/ for 550-more information.. Protection provided by MagicSpam 2.1-12.1 550 http://www.magicspam.com
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