Mail Servers

You do not need to host your domain email! Most domain registrars will supply an email package to suit and if required can forward to an existing mailbox. The thing to note is if you choose this type of package you will need to set up an MX record on the DNS servers to point back to their mail servers for it to work. An MX record is DNS entry specifically designed to point domain mail to a Mail Exchanger or server. 

You will need to make note of the MX record already existing before you change anything to do with the DNS servers at your domain registry. This may mean you will need to wait till the DNS entries have replicated through the Net. Go to www.network-tools.com and do a search for any email address on your domain even non-existent, jack@yourdomain.xyz will do. Then note the MX record when it appears. You can then point the DNS servers at the registry to the Dynamic DNS servers and then create an MX record at the Dynamic DNS service to point to the same Mail Exchanger as previous.

Now, if you DO intend to host your own mail then you do not need an MX record as long as the mail server is on the same PC (or IP Address on the Net) as the web server. As most Dynamic DNS services will set up two host entries automatically, one will be just the domain ie domain.com and the other will be www.domain.com to use for the web. It will be the first that your mail server will use. If the DNS provider creates only the www entry (unlikely) then you may need to create a separate host without the www part.

Mail servers come in various forms, styles, and price ranges! Essentially they do the same thing. Collect and distribute email. 

Depending on how you wish to set up your mail will determine how you run the mail server. If it is not going to run all the time then you can do the above by having the mail hosted via the domain registry or another host willing to host your email domain. The easiest way to collect it would be to have all domain mail forward to a POP3 account and have your mail server poll the POP3 mailbox at your leisure. Your mail server can do a sort to determine where the mail was originally meant to go and drop it into the appropriate mailbox.

The best way would be to leave it running and just let the Net send direct to your mail server, this way involves no 3rd party to host your mail domain and mail is sent/received virtually instantly. It also alleviates the need to create an MX record for it as mail will go to the domain address in the DNS regardless.