I did a search and could not find anything on this problem although there are 7 pages of people with this problem on the Comcast forum. It seems that under certain circumstances you can not get off the preview email page to see your emails on the Xfinity (Comcast) site. Here is my last post to Xfinity

"You can probably stop blaming Comcast. I am fairly sure the culprit is Zimbra. It seems that ‘Zimbra enterprise-class email’ has problem when linking with certain browsers and internet servers. For example, I can not get off the preview page using IE 9, Firefox or Google Chrome with either Windows 7 or XP in my office which has a slow link and an offsite server. This is with all cache and cookie cleared and only 3% of allowed usage. However, I can get into the email using Safari on my windows machine and the “Address Book” trick. I recommend anyone download Safari if they want to get into their email. At a Marriott, Zimbra works fine on all my browsers unless I go through a VPN and then I am back to only Safari. Until either Zimbra (Zimbra offers Open Source email server software and shared calendar for Linux and the Mac. ) fixes their software, or Comcast decides to change its service, you are stuck with what you have.
Suggest first try Safari, its free and only a slight pain it the A.
Change your email to Google or something else, bigger pain in the A.
Supposedly there is a new Zimbra release coming out soon, but who knows what it will do?"

ONe other item reported in previous posts is that at my home where I have Comcast internet service, I have no problems with email through either 1000T wired connection or n wireless.
Is there an answer to this problem from Zimbra?
All of us would appreciate any help from Zimbra however, you may not be aware of this.