Hi Tony,
We run our own content filter on all of our connections, so I have a suspicion that the two different systems might not be playing well together. There are also other oddities on our data network that could be the culprit, such as the fact we use shared public IP addresses (which can sometimes cause problems for VPN's) but without knowing more about exactly how the Message Labs proxy works or what it does, I'm afraid we're really outside of our scope of support here. Your internet access is clearly working fine, so it's a rather unique fault.
You mention that you're deploying these modems across your business. Are they on a business account, and have you been assigned a Data Manager? If so, I would recommend discussing with them the possibility of setting up a private APN on these modems. That will allow your own IT department to determine exactly what you can and can't access, and allow them to shape the network traffic to their own requirements, ensuring it works better with their systems (and would potentially negate the need to use a proxy in the first place).
If this is going to be a widespread deployment, then a bespoke package like the above would be the best way forward. Alternatively, the
setup guides for IT Administrators may help you to understand what our network needs to be in place in order for these kind of services to work properly, and shed some light on the problem.
Jon
eForum Team