Can’t send outgoing email when using a different ISP

You’re not a newbie–you know you have all your email settings configured properly in your email client (Outlook, Thunderbird, etc). In fact, you can send and receive email just fine from one ISP. But when you connect your laptop at a different location which uses a different ISP, all of a sudden you can’t send outgoing email. You get an error message like this one:

Send Message Error in Thunderbird

Send Message Error in Thunderbird

The issue:
In an effort to stop spam, many ISPs (Comcast, Cox, many others) block all traffic to port 25 on anything except their own SMTP servers.

The fix:
Replace the default SMTP Port 25 with 587. RFC 2476 explains why this works for newer SMTP servers. (For, the fix is to change port to 26.)

Alternate fix:
You will have to use your ISP’s default SMTP servers. Here is a tentative list that has not been checked for accuracy:

  • Earthlink: (To verify, visit
  • Comcast: (To verify, visit
  • Sympatico: or (To verify, visit
  • AT&T: (To verify, visit
  • Netzero: (To verify, visit
  • Charter Communications: (To verify, visit
  • MSN: (To verify, please check with your MSN provider)
  • RCNL: (To verify, visit
  • (To verify, visit
  • (CT, FL, GA, LA, NC, OH, RI, VA); (KS, NE, OK, UT, West TX); (AZ, CA, NV); (for customers
    in AR, ID, LA, MI, East TX, Tyler). Port 587 may also be required for SMTP. To verify, visit
  • Rogers: May require port 587 for smtp. To verify, visit
  • AOL: May require port 587 for smtp. To verify, visit

6 thoughts on “Can’t send outgoing email when using a different ISP

    • Hi John,
      It depends on which email client you’re using. Typically there is an “Email Accounts” option. Once you open that up and select your email account, there will be an “Outgoing/SMTP” tab where you can specify the SMTP port.

  1. Thanks, I used the info from your list to get my outgoing mail to work.


  2. Hi. Thanks from France. I had this issue when sending email from my PC using iPhone as a modem. I switched from port 25 to 587 and now it works fine.

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