Actually I am using e-mail for the first time on my phone, like I have one my official e-mail add which I want to use on my phone but I am just thinking that it is secure to use the E-mail on Phone

It is just as secure (or maybe more so, because of less malicious apps that can run on your phone) as on a PC.

So, if you are comfortable accessing your email from a PC through the Internet, then there's no difference if you do it from your phone.

More security to corporate or other email systems is possible with a VPN connection. If you use that on your PC, then it will be more difficult with a phone (Nokia's Mobile VPN client is difficult to configure, if you don't have a Nokia VPN gateway).

Let's start with a few scenarios:
1) Plain POP/IMAP emails
Connection is not secured, as the traffic from operator to servers through internet are not encrypted. On the wireless itself, GSM applies encryption.

2) Secured POP/IMAP with SSL/TLS
Secured end-to-end, you should use this if your servers support this.

3) Push mails
Mostly secured with propetiary/SSL/TLS encryption.