mail server setup problems

we are in the process of setting up a SL mail server and have pretty much everything running nicely, except mail.

our setup is as follows:
• previous email provider mx records are still active but have allocated higher priority to new server's mx record and according to the ISP those changes have since propagated;
• billion router as dhcp server for a shared studio space/network (same subnet for a variety of parties - we know not good, but since we need to share printers, etc. the only easy way to do so!), allocating a static IP to our new server with all necessary ports forwarded as well;
• DNS configured and running, does reverse lookups and added mail.example.com as A record;
• webmail works, i.e. we can login with a test users credentials;

we experience 2 problems:
1. incoming emails to an activated account get rejected with a 550 error message, still indicating our previous mail providers nameservers;
2. currently only SMTP seems to be working, i.e. we can send emails from external clients but strangely not from the server itself using mail.example.com (it does work using the servers internal IP address though);
3. if we telnet into mail.example.com from outside the studio we get connected, but internally again this only works with the IP address;

as we have trawled the net for some clues and unsuccessfully tried various things, we are beginning to wonder if something on the router is interfering.

any help or thoughts much appreciated.

ds.