메일이 발송되지 않을 경우
DNS MX 조회를 수행
Windows에서 DNS MX 조회를 수행명령어 nslookup -query=mx gmail.com
Linux/UNIX에서 DNS MX 조회를 수행명령어 host -t mx gmail.com
[handy:/home/handy]$ host -t mx nate.com
nate.com mail is handled by 10 smtp.nate.com.
[handy:/home/handy]$ telnet smtp.nate.com 25
Trying 117.53.114.15...
Connected to smtp.nate.com.
Escape character is '^]'.
220 ESMTP ARGUS Alpha 0.0.2 is ready to Serve. 2.2.2011011213
quit
221 2.0.0 closing connection
Connection closed by foreign host.
[handy:/home/handy]$ host -t mx daum.net
daum.net mail is handled by 10 mx3.hanmail.net.
daum.net mail is handled by 10 mx4.hanmail.net.
daum.net mail is handled by 10 mx5.hanmail.net.
daum.net mail is handled by 10 mx6.hanmail.net.
daum.net mail is handled by 10 mx7.hanmail.net.
daum.net mail is handled by 10 mx8.hanmail.net.
daum.net mail is handled by 10 mx9.hanmail.net.
daum.net mail is handled by 10 mx10.hanmail.net.
daum.net mail is handled by 10 mx1.hanmail.net.
daum.net mail is handled by 10 mx2.hanmail.net.
[handy:/home/handy]$ telnet mx3.hanmail.net 25
Trying 114.108.154.201...
Connected to mx3.hanmail.net.
Escape character is '^]'.
220 rmail-326.daum.net ESMTP welcome to HanMail.Net(tm) ready
421 4.5.0 XTOUT 61.74.67.182: Closing connection by timeout
Connection closed by foreign host.
[handy:/home/handy]$ host -t mx paran.com
paran.com mail is handled by 5 nmailex07.paran.com.
paran.com mail is handled by 5 nmailex08.paran.com.
paran.com mail is handled by 5 nmailex09.paran.com.
paran.com mail is handled by 5 nmailex10.paran.com.
[handy:/home/handy]$ telnet nmailex07.paran.com 25
Trying 211.41.82.136...
Connected to nmailex07.paran.com.
Escape character is '^]'.
220 nmailex07.paran.com ESMTP
quit
221 nmailex07.paran.com
Connection closed by foreign host.
[handy:/home/handy]$ host -t mx gmail.com
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
[handy:/home/handy]$ telnet alt1.gmail-smtp-in.l.google.com 25
Trying 74.125.95.27...
telnet: connect to address 74.125.95.27:Connection refused
정상적일 경우
Connected 라고 나오고 텔넷으로 연결이 된다.
하지만 문제가 있을 경우
Connection refused 라고 나온다
