Problem sending emails containing sites.google.com links

I'm having a problem sending emails containing links to Google Sites. I've tried to narrow down the problem in a number of ways. The problem occurs when I'm sending the email from a client app (not the xfinity web site) using a comcast.net account. I've seen it happen with both Apple Mail (on Mac and iPads) and a friend has seen it using Outlook on Windows. The outgoing mail server is set to smtp.comcast.net. Here are the symptoms:

- When I send the email to a non-comcast account (e.g., my gmail account) it just never shows up. It does not even appear in my gmail spam folder.

- When I send the email to my comcast account it does not show up in my inbox on the xfinity mail web site, however it does show up in my spam folder there (with the link crossed out when I look at the message) and it shows up in my sent folder.

Here's how I believe the problem can be reproduced:

- Send an email via an Apple Mail client using a comcast account with the outgoing mail server set to smtp.comcast.net. 

- The outgoing message contains the string "http://sites.google.com/view/", either in the message text or as the target of a hyperlink. That URL in itself is not a valid sites.google.com link but it is enough to trigger the problem. I tried a number of valid sites links and they all have the problem too. Note that if the message text contains just "sites.google.com/view/" as plain text (no "https://" and no hyperlink when sent from the client side) there is no problem.

This problem started happening perhaps in the last couple of months. Emails containing google sites links sent via a mail client using a comcast.net account used to work. Also, sending emails containing google sites links via the xfinity web site directly (no mail client app) does still work.

Thanks,
Sharon