If your Outlook successfully sends emails from your home network, but fails to do so on certain wireless networks, this may be because the network port 25 is blocked by the internet provider you are using at the time. If you are seeing this problem at work, your company may allow sending emails only on the wired ethernet network, but prohibit it on the wireless network for security or monitoring reasons. In certain public wireless network, the use of port 25 may also be blocked to prevent abuse through spamming or other illegal activities. Furthermore, many internet server providers (ISP) may block access to the SMTP port, because they reserve it for mail servers to relay emails to each other and to block widespread worms and viruses.

Solution

Many email providers, such as Yahoo, Google and Hotmail provide port 587 (SMTP email submission) as an alternative port to send emails on. If you are having problems sending email through port 25, try changing it to port 587 with the following steps:

1. Select Tools from the main menu
2. Click on E-mail Accounts… or Account Settings… to open the Internet Accounts window
3. Click on View or change existing e-mail accounts and click Next
4. Select the email account you wish to change and click Change
5. In the E-mail Accounts window click the More Settings… button
6. Select the Advanced tab
7. Change the setting Outgoing server (SMTP) from 25 to 587
8. Click Ok to close the properties window
9. Click Next and Finish to save your settings

Your email provider will likely require authentication for your outgoing email as well, so make sure that you setup Outlook with the correct user credentials for your email account.

Other Considerations

If your Outlook is still unable to send emails through port 587, contact your ISP or local network administrator to find out if another port is provided for sending emails

If available, you may also consider using Secure SMTP through SSL (port 465), the IMAP protocol (port 143) or the Secure IMAP protocol through TLS/SSL (port 993)