Outbound Port 25 Blocking (OP25B)

This page explains about the outbound port 25 blocking, or OP25B.

※ You will need to change your setting to avoid negative impact from the OP25B implementation. Please refer to information below.

Dealing with OP25B

Measures to Prevent Spams

As a step to improve the security of our Internet service and our members, Asahi Net has implemented Outbound Port 25 Blocking (OP25B). This would reduce extensive amount of Spams, or unsolicited bulk email (UBE) as well as virus-infected emails.

What is OP25B?

Outbound Port 25 Blocking (OP25B) is a measure to prevent any intentional or unintentional sending of bulk unsolicited email through an Internet provider's outbound email server. Many of those spams are directly routed using the port 25 to the servers of recipients. By blocking the port 25, it can limit the use of port 25 which accordingly block the flow of spams.

What it is

The Merit of Outbound Port 25 Blocking

If your computer is infected with a virus or being hacked by malicious third party, your computer will be taken over and become as a hub for unsolicited bulk email sending in which act can risk your personal identity information.

OP25B prevents such unintentional bulk sending of spams and leakage of your personal information.

OP25B affected Internet Connection Service and Access Point

All Asahi Net users using IPv6 connection will be affected by OP25B regardless of which service course you are contracted.

If you are connecting with IPv4, please note that OP25B is implemented in the following manner. Please also refer to our informational page Dealing with OP25B

  Currently enrolled service courses using IPv4 Other Company’s Active Outbound Email Server
Service Course

■Fiber-Optics (FTTH)

  • Asahi Net FTTH with FLET’S
  • FLET’S Hikari Next
  • FLET'S Hikari Light
  • Asahi Net docomo Hikari


  • BB Entry

Starting as of April 5th, 2017, we progressively apply OP25B to all service courses.

After the implementation, you will no longer be able to use the port 25 to send emails. Please refer to Dealing with OP25B to refer to its step by step guidance


  • Asahi Net WiMAX2+
  • Asahi Net LTE(ANSIM)


  • Dial-up

Subjected to OP25B.

You will no longer be able to use the port 25 to send emails. Please refer to Dealing with OP25B to refer to its step by step guidance

Although outbound mail server port 25 is still available to use by choice as long as you are enrolled in one of our Internet connection services, we highly recommend to accord with Asahi Net’s recommended configuration for your online safety and convenience.

Please check the current configuration status on your email clients.

Email Client Configuration