Some Internet service providers (ISPs) have recently started blocking the standard sendmail port (25) to help prevent direct to MX spamming, and abuse of open proxies/relays from their dialup and broadband clients.

Unfortunately, a side-effect of this practice is that it prevents users from accessing our SMTP servers. If you are getting error messages when you try sending mail which state something to the effect that smtp.yourdomain.com is not responding or not found, contact your ISP to find out whether they are blocking port 25. If they are, you can probably send outgoing mail through their outgoing mail servers (contact your ISP to find out what outgoing mail servers you need to use).

However, sometimes an ISP will not allow e-mail to be sent through their SMTP server with a different domain. In other words, you can only use their SMT server if you have your reply address set to your ISP based e-mail address, not your custom domain name address. If this is the case, installing this site application, and configuring your e-mail client to use the alternate port will allow you to use your own custom domain name e-mail addresses.
Some ISPs Known to Block Port 25

Below is a list of some of the major ISPs that are known to block port 25. While this list is a good starting point, it is far from complete and is prone to change over time. Ultimately, the most reliable way of knowing whether your ISP is blocking port 25 is to contact them directly and ask.

ISP SMTP Server
AOL smtp.mail.aol.com
Adelphia mail.adelphia.net
Ameritech mailhost.det.ameritech.net
AT&T mailhost.att.net
AT&T Global smtp1.attglobal.net
Bell Internet High Speed (Canada) smtp10.bellnet.ca
Bell south mail.bellsouth.net
CableOne mail.cableone.net
Charter Communications smtp.chartermi.net
Charter.net smtp.charter.net
CharterTN.net mail.chartertn.net
CharterMI.net mail.chartermi.net
Cinncinnati Bell/Fuse Dial-up smtp.fuse.net
Coax.net smtp.west.coax.net /east / central
Comcast smtp.comcast.net
Cox smtp.west.cox.net; smtp.central.cox.net; smtp.east.cox.net (either of the three depending on your location).
Earthlink smtp.earthlink.net
Etisalat smtp.emirates.net.ae
Epix out.epix.net
Erols smtp.erols.com
Frontiernet.net smtp.frontiernet.net
Fuse smtp.fuse.net
Juno smtp.juno.com
Mindspring smtp.mindspring.com
MSN smtp.email.msn.com
MSN.DSL secure.smtp.email.msn.com
NEBI.com mail.nebi.com
Netcom smtp.ix.netcom.com
NetZero smtp.netzero.net
OOL (Optimum Online) mail.optonline.net
Pacific Bell (Pacbell) mail.pacbell.net
Panix.com mailhost.panix.com
PeoplePC smtp.peoplepc.com
Quixnet.net smtp.quixnet.net
RCN smtp.rcn.com
Road Runner (NYC) smtp-server.nyc.rr.com (if outside the NYC area, please call your local Road Runner Support team)
Rogers Hi-Speed smtp.broadband.rogers.com
SBC smtp.sbcglobal.net
SBC Yahoo smtp.sbcglobal.yahoo.com
Speakeasy mail.speakeasy.net
The-Beach.net mail.the-beach.net
Verio smtp.veriomail.com
Verizon outgoing.verizon.net or smtpout.verizon.net

How to Check if your ISP blocks SMTP

Windows Users

1. Click Start from your Windows Taskbar.
2. Click Run... from the Windows Start menu.
3. Type telnet smtp1.arin.net 25 into the Run... dialogue box.
4. Press Enter or Return.

Mac Users

1. Launch Terminal.
2. At the $ sign prompt, type telnet smtp1.arin.net 25.
3. Press Enter or Return.

Linux Users

1. At the command line prompt, type telnet smtp1.arin.net 25
2. Press Enter or Return.

if you get back 220 smtp1.arin.net . . . your ISP DOES NOT block port 25.

If your connection hangs, fails, or returns something other than smtp1.arin.net then your ISP IS blocking or redirecting port 25 to their own server.