I don’t believe its possible to access Meridian Mail from external systems as it is not exposed through an interface. Only product I’ve seen which allows complete utilisation of the mail ports is Nortel Symposium (surprise, surprise!) I guess it may be possible by writing low level code to access the core services over the embedded LAN but I’ve never heard of anyone doing it.
Even with Symposium, its only via the Symposium scripts that the Mail can be accessed. I’ve gone through the TAPI manual and it doesn’t look as if anything is exposed but can mail this to you if you want.
Nortel don’t tend to expose a lot of the Meridian functionality eg. Mail, ACDDNs (hence incall key issue) guess its competitive advantage or something.