An MX record sets the mail server for a given domain name, directing all email to the correct location and mail server. If you have the domain myname.com, setting an MX record for myname.com will determine where mail for firstname.lastname@example.org is sent. The mail server must be configured to accept mail for the domain name.
By default your domains with Eco Web Hosting will have an MX record pointing to mail.(your domain). You can change this if you wish to use a different mail server. To do this you need to log into your domain control panel and select Manage DNS if that is available to you.
These MX records point "mail.(your domain)" to the mail server assigned to your domain so when setting the mail box up in your email client, you will be able to use "mail.(your domain)" as the incoming and outgoing server. Your username will be the mail box name and your password will be your mail box password.