can't telnet mail.mydomain.com!

but i still cant telnet port 25 to the mail server from outside, my question is.. how would i know that my MX record got propagated or not yet ?

Telnetting to the port has no bearing or dependency on MX records. MX records are only used by other mail servers when trying to send mail to your domain. Your ability to get to your server is not related to MX. That said…

when I try to host from outside mail.mydomain.com it will resolve my static IP address

That indicates that your DNS has propagated.

but when i try to ping mail.mydomain.com, it it doesn't ping, is that normal or it should ping from outside ?

Typically you should be able to ping, but there are many reasons why ping may fail - any number of devices routers, firewalls, etc.) between you and the destination may be blocking ping traffic.
A better test would be traceroute, and you can use links at http://www.traceroute.org/ to trace back to your IP from many places around the world.
If you're resolving the correct IP address, though, I'd guess that the problem is just your router not responding to PING,

Beyond all that, though, you should test the other ports beyond 25. Many, many ISPs block port 25 in an attempt to prevent people running servers on their link under the guise of wanting to limit spam originating from the network.

This is all speculation though since you're asking us to work in the dark.
If you'd reveal your actual domain name other people here could test and verify the above theories.

I can't help with the local connectivity testing, but the DNS is showing errors. The reverse DNS and the forward DNS and the MX do not match. That will cause receiving mail servers to ignore messages originating from your mail server. Specifically, your reverse DNS is mismatched.

Definitely test connectivity with a port other than 25. That can get blocked by ISPs. Open up something on a port above, say, port 1024, and try to connect into that. Potentially via port-forwarding at a sufficiently-capable external firewall, for instance, and redirecting the connection to, say, port 80 on the server.

From what I can see, mail for usabilitymatters.com should be handled by the host mail.usabilitymatters.com which has the address: 66.207.196.178

There does not seem to be a functioning mail server at that address, though.

Certainly, I can connect to port 25 on the server, but I don't get the typical mail server greeting. Indeed, the connection automatically closes soon after it's established.

So things to check are:

1) Is that the IP address you expect?
2) If it is you're going to have all kinds of problems with mail from this address since the reverse DNS does not match (66.207.196.178 resolves to 66-207-196-178.beanfield.net - if beanfield.net are your ISP then talk to them about setting up your reverse DNS).
3) If that is the right address, check your router/firewall settings to ensure that incoming connections on port 25 are being routed to your mail server (you are running in a NAT network, right?)

In Server Admin, you should look at the SMTP server logs to see if it sees an incoming call, and if so, what error message it generates.

There may be configurations that will kill the connection right away if you're coming from a bad IP address for instance. (normally, postfix postpones disconnection until you get to the DATA phase, but there are ways to get it to act right away).

2) Yes beanfield is my ISP and i will call them to setup the reverse DNS.. but my question, what the reverse DNS will help me with ?

It will help you with spam.
90% of the big ISPs will currently reject mail from your server.
When your server connects to them (to send out mail), the connection goes something like this:

You: Hi Mr Big ISP, I'm mail.usabilitymatters.com and I have some mail for you
ISP: Hi there. You say your name is mail.usabilitymatters.com but if I reverse lookup your IP address I get 66-207-196-178.beanfield.net which doesn't match your claim, nor even your domain. As a result I don't believe you so I'm not going to accept any mail from you. Goodbye
<connection closes>

By having reverse DNS that matches your server name, remote mail servers are more likely to accept your message.

but still cant telnet mail.usabilitymatters.com 25

are you checking from inside your network? or remotely?

now you mentioned that u connect to port 25, and u didn't get the typical mail server greeting.. how can i set that up and how to test it as well to see if i get the greeting msg!

the mail server is issuing a standard greeting now. I'm not sure what would have prevented this before:

$ telnet 66.207.196.178 25
Trying 66.207.196.178…
Connected to 66-207-196-178.beanfield.net.
Escape character is '^]'.
220 server.usabilitymatters.com ESMTP Postfix

I do note, though, that your server is identifying itself as 'server.usabilitymatters.com'. This name should match your MX record and the reverse DNS for your public IP. I'd change this to 'mail.usabilitymatters.com' given the previous discussions about DNS.
Note this only involves changing the name the mail server uses to identify itself, and it separate from the hostname of the machine. You can change this via Server Admin -> Mail

but the reverse dns for that private ip address resolves only server.usabilitymatters.com not mail.uabilitymatters.com

The internal DNS/client shouldn't be affected since they won't be receiving SMTP mail from this server. Internal clients should be able to connect with either hostname without an issue.

one last question, to setup a webmail server and to access it from outside.. because i already have the wiki goes to server.usabilitymatters.com, what is the easiest way to take ONLY the webmail externally!

I take it you mean you want the wiki to be internal only, while the mail server can be accessed from both inside and outside the network?

If that's the case there are several possible solutions. I would probably setup a second virtual host in Apache, running on a different port number (e.g. 8000). For this virtual host configure only the webmail and disable the other services.
Then setup your port forwarding in your router to forward web traffic to port 8000 rather than port 80. Now when external hits come in they'll hit the virtual host that knows about webmail but none of the other services.

Now all you need to add is an SPF record indicating your IP is authorized to send e-mail. The most anal retentive hosts out there (aol.com to be specific) require the server name to match DNS, forward and reverse, AND have a valid SPF record in the domain. Your SP should be able to set that for you (it its not there now).