退信提示：“UnKnown can't find abc.com： Non-existent domain”
退信提示：“SMTP error，DOT：550 abc.com has no A,AAAA,,or MX DNS records…”
退信提示：“SMTP error, MAIL FROM：553 SPF check refused(softfail)”
退信提示：“SMTP error, DOT： 554 5.7.1 This message has been blocked because...”
退信提示：“No MX or A records for abc.com”
退信提示：“Host abc.com(xxx.xxx.xxx.xx) RCPT TO said 421 Refused. ”
退信提示：“Host abc.com(xxx.xxx.xx.xx) MAIL FROM said 451
退信提示：“550 Sender domain must exist ”
