I have been pulling my hair out trying to get my Mac Mail to receive with Netidentity. It will send, but not receive. If you have been having this issue, here is a solution I got after being on hold with them (Netidentity) for 32 minutes. They have upgraded and now I cannot get mail using Mac mail and they state that there no changes needed in the client, but alas...
Open preferences in Mac Mail, and click on the tab "advanced". Under "Authentication", select "Password", close the window and save. That's it. I spent a long time trying to jiggle all the other settings. I hope this helps someone.