Thank you! I think I found the issue - the new phone system was installed configured on vlan 200, but the smtp settings were copied over from the old system which was on the default vlan 1 so what was happening was that the voicemail system was contacting the smtp server via the vlan 1 address (routed through the gateway) yet the server was also listening on vlan 200, and although the replies from the server seemed to go out on the vlan 1 interface there was some issue, either timing or routing or route confusion. Switching the voicemail system to contact the smtp server directly on vlan 200 seems to have resolved the issue (as usual, time will tell).
Chris
