A number of my clients have sent me emails that have not come through, I am using Outlook Express for my mail and am not running any other internet security software.
1) The emails have not been returned to them as undelivered.
2) The emails do not show in any of my mail folders.
3) Their email addresses are on the allowed list under configure preferences,
4) I have received other emails from the same clients, this seems to be intermittent.

Help/advice would be much appreciated, thanks.

Operating System:Windows XP Pro
Software Version:7.0
Product Name:ZoneAlarm Internet Security Suite