Bila anda tidak dapat mengirim email dari ISP, direject saat menggunakan SMTP/pengiriman email, maka kemungkinan IP dari ISP yang anda gunakan masuk daftar blokir/RBL. Definisi RBL = “Real-time Blackhole List”.

Jadi semata-mata karena ISP yang digunakan masuk RBL sehingga persoalan terblokirnya IP dan tidak dapat melakukan pengiriman bukan karena kesalahan pada server.

Penyebab IP diblokir tentu sangat beragam. Informasi dibawah ini semoga dapat membantu menjelaskan:

BRBL: Sorry, your email was blocked

We are sorry you have reached this page because an email was blocked based on its originating IP address having a “poor” reputation. The “poor” reputation may have been caused by one of the following reasons:

* Your email server contains a virus and has been sending out spam.
* Your email server may be misconfigured.
* Your PC may be infected with a virus or botnet software program.
* Someone in your organization may have a PC infected with a virus or botnet program.
* You may be utilizing a dynamic IP address which was previously utilized by a known spammer.
* Your marketing department may be sending out bulk emails that do not comply with the CAN-SPAM Act.
* You may have an insecure wireless network which is allowing unknown users to use your network to send spam.
* In some rare cases, your recipient’s Barracuda Spam Firewall may be misconfigured.

Bila anda gagal melihat website, periksa terlebih dahulu dengan tools online seperti:

http://host-tracker.com/
http://siteuptime.com/users/quickcheck.php
http://alertra.com/

untuk memastikan website online. Bila hasilnya website online/OK dan anda tidak dapat akses ke website, kemungkinan IP ISP yang anda gunakan ter-blokir firewall pada server. Karena beberapa sebab, pengunjung tidak dapat melakukan akses ke website karena IP yang digunakan ter-blokir oleh firewall pada server. Sebab utama IP berada pada firewall antara lain karena:

Gagal login (kemungkinan user dan atau password salah). Gagal login mencakup login cpanel, ftp, email, dll.

Contoh log firewall:
xx.5.8.146 # XX: 10 (pop3d) login failures from xx.5.8.146 in the last 300 secs - Wed Oct 22 21:08:17 2008
xx.229.57.152 # XX: 5 (sshd) login failures from xx.229.57.152 in the last 300 secs - Thu Oct 23 00:34:31 2008
xx.124.221.51 # XX: 5 (cpanel) login failures from xx.124.221.51 in the last 300 secs - Fri Oct 24 08:36:56 2008
xx.165.169.165 # XX: 5 (mod_security) login failures from xx.165.169.165 in the last 300 secs - Mon Oct 27 14:40:50 2008
xx.44.149.119 # XX: 5 (smtpauth) login failures from xx.44.149.119 in the last 300 secs - Sat Oct 25 11:08:19 2008
xx.81.125.222 # XX: (PERMBLOCK) xx.81.125.222 has had more than 4 temp blocks in the last 86400 secs - Mon Oct 27 15:31:26 2008
untuk alasan tertentu, IP awal kmi ganti dengn xx.

Guna menghindari IP anda masuk firewall pada server, pastikan:
- user dan password tepat
- gunakan smtp authenticaion (setting pada email client)
- script pada website berfungsi baik
- periksa komputer dari kemungkinan virus, spyware, dll.

Lihat juga tulisan:
Server down?
http://www.tadulako.com/bloghosting/2006/08/30/server-down-could-not-locate-remote-server/

Perlu menjadi catatan, tools diatas (siteuptime, host-tracker, alertra.com dan lainnya) terkadang memberikan laporan yang tidak akurat bila digunakan pada website dengan server hosting yang berlokasi di Indonesia.