Overview
The following describes how port 25 blocking works and a few steps to help you resolve it.
What is port blocking?
Email is transmitted from a specific port number. Mail submitted via SMTP traditionally defaults to port 25. The problem is that port 25 is frequently blocked by Internet Service Providers (ISPs) as an anti-spam technique. To send email on such ISPs, one must either:
- use SMTP to send — the recommended option to send email from DreamHost is to use use port 587.
- use the ISP's own SMTP server — this may require you to use the ISP's domain to send email.
List of ISPs
The following is a list of ISPs that block port 25, including the addresses of their SMTP servers:
-
ISP SMTP Server Cox smtp.cox.net EarthLink smtp.cox.net Grande Communications mail.mygrande.net Optimum Online mail.optonline.net Spectrum mobile.charter.net xfinity smtp.comcast.net
Port 25 at DreamHost
DreamHost blocks port 25 for outgoing SMTP calls on shared servers (this is not a restriction on VPS/Dedicated Servers).
This often happens when sending from a mail form or custom mail script while attempting to connect to an external mail host (not a DreamHost mail server).
Resolving this issue
Change your outgoing port
The easiest solution is to configure your email client to use 587 with STARTTLS.
Use the ISP's SMTP server
If you are seeing the error message "smtp.example.com is not responding or not found", contact your ISP and find out if they're blocking port 25. If they are, you can probably send outgoing mail through their outgoing mail servers.
Use PHP SMTP mail
If you're using PHP to send email, see the following articles for full information on using SMTP.