This can be caused by several different issues:
1.) Mail server issues
2.) Mail client issues
3.) Internet issues
4.) Connectivity issues
Best way to solve this is by elimination.
From your browser go to http://mail.skywaves.net or http://mail.madriveraccess.net [depending on which cluster your mail lives on.] If you get a login screen you know that #3 and #4 are not the problem. If you do not get a login screen can you surf the web? If not the issue is with your Internet connection/computer/etc
Can you login to the mail server? Is any mail present? If you send yourself an email from gmail, yahoo, etc does it arrive? If it doesn't arrive one thing to check is if your domain name has expired. Please go to http://internic.net and enter your domain name. Confirm it hasn't expired. If it has let us know and we can assist.
If the web client is getting email but your Outlook, etc is not - confirm your Outlook settings. We have Knowlegebase articles on the how to. If you can receive but not send change your outgoing SMTP port to 587. Internet providers are blocking outgoing port 25 often.
Hope this helps!