Using an antivirus scanner to scan your email is not necessary if the scanner is also configured for autoprotect since Outlook saves all attachments to a temp folder before opening and the scanner will detect a virus at that time. In many cases, you can easily recognize potentially infected messages and will delete them without opening them, with or without an antivirus scanner.

Autoprotect, updated virus definitions, blocked attachments, and the latest patches will do more to protect you than using an AV scanner on the email you send and receive.

Email scanning, like autoprotect, is only as good as the most recent virus definitions. Email scanning also slows down your send and receives as it scans each message.

If you change the blocked attachment settings in Outlook, only unblock the attachments you use regularly. If you can't access the attachment, neither can scripts or iframes.

It's also a good idea to make sure your OS and Internet Explorer has all the latest security patches installed. If you don't use Window's autoupdate feature, visit Windows Update once a week.

Also note:
Scanning your outbound mail won't prevent viruses that use their own SMTP from sending mail.

Some older antivirus scanners change the mailserver names to "localhost" or similar names and cause problems sending and receiving messages.