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For months I've been using a 5g sprint card for my laptop to check email. I have gmail, and three other accounts all configured in Thunderbird. Recently, I got AT&T internet service at my house and I discovered that I cannot send email when connected to my home service. I can receive just fine, but I can't send. If I plugin my 5g card, I can sent/receive with no problems.

At home I'm connected wirelessly through a router. What could be causing this interruption? I even tried pinging my smtp server only to see it timeout when connected via AT&T. If more details are needed, I'll be happy to share.

Correction: Sending through Gmail's smtp works. What types of things should I look for to diagnose the issue?

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Most residential ISPs restrict outgoing connections on port 25: Only connections to the ISP's SMTP server are allowed, any other outgoing connections on port 25 are blocked.

If your third-party e-mail is using an SMTP server on port 25, see if you can use other ports instead. (This GoDaddy support page suggests you can access your SMTP server on ports 25, 80, or 3535 without SSL, or on port 465 with SSL.)

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Some ISPs limit access to their outgoing SMTP servers to IP addresses on their network. Google doesn't. If you are attempting to send to Sprint's SMTP server from AT&T that may be your problem.

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I'm sending to a third-party SMTP, hosted with GoDaddy. –  Jonathan Sampson Mar 23 '10 at 3:32

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