Outlook Express

(1) The first step is to open Outlook Express (using Outlook Express as an example), then
you select the "tools" option on the top tool bar, then the "accounts" option.  See image.

(2) Next, we will ensure that we are viewing our mail accounts, and we will select the mail
account that we have configured in our email client Outlook Express.  You will select
the properties tab on the right of the screen.  See image.

(3) Next, we will select the servers tab at the top of this section, and we will confirm that
the incoming mail server is set to mail.yourdomain.com and the outgoing mail server is
specified as mail.yourdomain.com, you will next ensure that your account name reflects
the combination of your userid and domain name so in your case,

YourUserName@YourDomain.xxx

Ensure that you have your complete username and domain name there. 
Not just username. Example: janedoe@acmewidgets.com

IMPORTANT!
If you desire to also send mail out thru this server, you will
also place a check mark in the "My server requires authentication" in the Outgoing Mail Server area.
Once checked, select Settings. See image below.

(4) And finally, in order to send mail, the server needs to confirm that you are allowed to send
mail thru it so spammers cant use all your resources to send spam thru your server.  This is
done by configuring your mail client to send information to the server that includes your
username and password to authenticate and validate you.  You will specify by placing a
check mark on "Use same settings as my incoming mail server"