There are a couple of issues that come to mind. I've seen this a few times.

I'm a little unclear on part of the issue. Are the messages going to the mailbox, and you're not getting the email copy, or are they doing neither?

The first issue that comes to mind is a shortage of space on the C: partition on the Axxessory Talk if it's the email that is being lost. On large wav files we've seen this happen when over time, the registries on the Windows box get bigger and bigger and space for the temp file depletes even though there is plenty of space on the other partition.

The second problem seen a LOT on email delivery of the wav files when using POP3 is the firewall/router gateway setting. 100Mbps, full duplex will cause problems that will have you chasing your tail for months. On our installations we set up a DMZ and allow the office LAN to run at that speed, but then put the Axxessory Talk off to one side and configure a seperate firewall to run at 10Mbps, half duplex for the locations that use POP3 for delivery.