Linksys solution:
This problem is usually caused by an authorization problem. To resolve the issue, please follow these instructions:
1. Open your browser and type
http://192.168.1.1/Filters.htm into the "Address Bar".
2. You will be prompted for a username and password. By default there is no username, and the password is admin. Once you type in the password click OK
3. Once you've gotten past the log in you should be at the filters tab.
4. Click on the Forwarding tab
5. Once the "Forwarding" section opens, click the Port Triggering button.
6. Once you've done this, a new window will appear. Here is an example of how to fill out the ports:
Application Name
E-mail
Trigger Port Range
25 ~ 25
Incoming Port Range
113 ~ 113
7. Once you've done this hit Apply, then Continue.
8. After the settings have been applied, you should be able to use e-mail normally.