If after upgrading to Avast! Antivirus 7.0.1426 causes emails not to send from Microsoft Outlook, try the following steps as a work-around:

1. Close down Outlook
2. Open avast! user interface by clicking on the icon in your system tray (near clock)
3. Under Real-Time Shields (on the left), click Mail Shield.
4. Un-check “Scan outbound messages”.
5. Click the “Stop” button. Wait 30 seconds.
6. Click Start Button.
7. Open Outlook and see if your message sends.

Source: My experience on Windows XP Pro SP3 and Microsoft Outlool 2003 SP3. GoDaddy’s outgoing mail server (smtpout.secureserver.net) was involved and still gave me problems after the avast! setting changes. I switched to my ISP’s outgoing mail server  smtp.knology.net. The combination between the mail server and the avast! changes got me back up although I’m not completely sure why.