Some ISP's are blocking port 25. If you are using (Example) AOL as your ISP and you are trying to check your gmail.com account using Outlook Express, then you have your settings set wrong and it will now send any emails. You need to correct the settings.
Go to http://www.google.com
and type in
my ISP & Outlook Express or
my ISP & Thunderbird
and it will tell you step by step procedure for setting up your email account. It's important to follow each and every step, because not following 1 step will not allow you to send and/or receive.
Here are some interesting links to read
http://members.aol.com/adamkb/aol/mailfaq/imap/
http://www.aota.net/Troubleshooting/port25.php4
http://www.mailsbroadcast.com/email.broadcast.faq/33.port25.blocked.email.faq.htm
Hope this helps

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