SMS E-mail from Mobile (O2)

I am trying to send an SMS-email from my O2 phone.

I get to the point where I have entered the message and recipient but then it asks for the E-mail server number

What is this number and how do I find out what it is ?
(Do I need to phone O2)

Do people find that SMS-email works ?
and how much does it cost to send a message (the cost of the call to the email server ?)

thanks.

I use the service provided by Go2Mobile.ie. The email server number is +353-86-6055506. The service is free, other than the normal text message cost. I've found it to be very reliable. The only problem I've found is that MS Outlook seems to flag some of the incoming messages as spam.