This problem occurs sometimes when you are using your laptop on other networks than your own. If you use webmail you will not have this problem occur. This problem can occur with people who use Outlook or Outlook Express on their laptops and use at different locations.
Spam is a large [problem with email today. To stop their systems from being used to send spam some Internet providers are not allowing email to be sent on their networks using the sending mail service on another provider.
The problem is called Port 25 blocking. Ports are used on networks to let certain traffic thru, sort of like ports where a ship stops and unloads. Only stuff that belongs to that ship is sent to that port to get on that ship. Port 25 is used for sending email. Web pages use port 80.
To get around this problem there are 2 solutions. The first is the easier. If you are traveling then use the webmail account that your Internet provider supplies and you can check and manage your email thru it. Many people (including me) now only use their webmail account as it is easy to check and do email on any PC thru the web browser.
If you want to use Outlook or Outlook Express on your computer even when traveling then you will need to go in the account settings if you encounter this problem. You will need to know what Internet provider is supplying Internet at your location and what their SMTP (the service Windows use to send email) name is which will look like mail.att.net Then you will need to go in your account find the setting for SMTP service name and record what the name is currently and then change to the new name. You should then be able to send and receive emails. After you leave that location you will want to go back in and change the SMTP provider back.
Thanks Lynn for the question.