I need to setup email for myself.
Bind has been configured with an MX record.
Port forwarding for port 25 have been setup to go to the ip address of the server.

Some background:
Centos release 5.3(Final),sendmail and dovecot installed for email
Firewall on server is open for STMP (25) and DNS (53).

Problem:
I went to Dyndns.com and attempted to test email using my MX that on my server but the error I get from them is that I need an A or AAA record for the MX (which I have). I have never setup my own email before so but this is fun/frustrating at the same time.

All the examples, tutorials online talk about the server side configure, but not how to tell you domain hosting company to point your domain mx record to your server.

In the control panel on your web host, you should have advanced DNS settings - you need to enter the MX record here. I presume this will be your dydns name - I've never used this, so not sure of the format, but it will be along the lines of 10 yourown.mx.server.com

So say your mail goes to you@acme.com. The path will be sender > acme.com > looks up mx record pointing to dydns > your ip address, which is forwarding traffic on port 25 to sendmail

And then you should have mail

PS - You should not open the DNS port on your firewall inbound, this is not necessary to recieve mail when MX records point to an external source. Misconfigured, open DNS servers are prime targets for abuse.

Email has been the hardest part for me.

No examples work, the manual I have says do this and that but nothing works regarding email.
So far I know that whomever host my domain (example: nonodomain.com) is where I need to tell them about my MX record for example: nonoemail.com
Guess I need to tell them my the IP of the server I want to use to receive email using dovecot (ip: 123.456.676.12)
Not sure if I need to port forward anything to my server.
/etc/dovecot.conf (from my reading) says I need only change the following: #myhostname = D787.nonodomain.com, #mydomain =nonodomain.com, #myorigin = $mydomain, and uncomment "inet_interfaces = localhost", #mynetworks = 123.456.676.0/28, 127.0.0.0/8
With that said I should be able to sent email to cane@nonodomain.com and the user named "cane" on the server should have mail. I don't know whatever to think of-but it's all new to me so I shouldn't be too frustrated. I feel like I'm stupid or something-thanks guys.

How does my server know to listen for email that comes in for it?