I would like you to assist you regarding your email issue.
If you are opening an email using Browsers .Check the below settings.
Open Internet Explorer>Tools>Internet Options.
Under the Connection tab.Check for use proxy server check box
Make sure the box is unchecked.
If the box in check please uncheck it and click OK
Click on Advanced tab and click on reset.
Try to open your email now.Hope this will help.
If you are using Email Clients like,Microsoft Outlook,Windows Live Mail etc then
Check that your user name and password is correct.
Check the mail incoming server and out going server settings.
Check the port settings.
Refer the link for more server settings details:http://customer.xfinity.com/help-and-support/internet/email-client-programs-with-xfinity-email/