Nokia Messaging is a 'glue' between your e-mail server and your device. This 'glue' runs on top of Ovi (Nokia). You provide this service e.g. your IMAP username/password, and Nokia Messaging will 'push' the e-mail to your device even if your e-mail server officially doesn't support 'push' e-mail. I don't know whether the connection between Nokia Messaging and Ovi is encrypted though.

I use IMAPS on my E71 with the native client (not Nokia Messaging) and I receive almost immediately new e-mail, and have configured this to be used only on 'my own network' which I assume means only when I am not roaming. I also would say that not every e-mail box needs 'push'. It isn't meant for every person or every purpose. But since you ask for it here, you probably have a need for it.

Also see other threads such as Email app like Thunderbird.