Can't send email through Comcast SMTP to my domains

Question

I am a Comcast customer with 3 computers and 3 computer users in the house. There are 2 fully updated Macs and one PC running Windows 7. We use Mail on the Macs, and Outlook on Windows 7. All computer accounts are configured to send mail through port 587 of smtp.comcast.net.

I also have two personal domains registered with Network Solutions. For the sake of this discussion, call my domains myOwnDomain1.com and myOwnDomain2.com. I have email addresses at both domains. They are of the form [e-mail] and [e-mail].

Until recently, our email worked as expected. However, sometime between September 13, 2012 and September 19, 2012, we lost the ability to send email through Comcast's SMTP server to the email addresses at my personal domains.

If we attempt to send email through Comcast's SMTP to those addresses, the email never arrives. Furthermore, the email clients give no indication of failure. The email just never arrives. The result is the same on all 3 computers and with all accounts on those computers.

We can successfully send email through Comcast's SMTP from any of our accounts on any of our computers to any email address other than to my email addresses at my personal domains! However, I receive email at those domains that is not sent through smtp.comcast.net. For example, I can successfully send email from my gmail and yahoo accounts to my email addresses at my personal domains. Furthermore, I can successfully send email through smtp.myOwnDomain1.com to [e-mail] and through smtp.myOwnDoman2.com to [e-mail].

Comcast says the problem must be at Network Solutions. According to Network Solutions, their logs show they are not blocking reception of the email, and our IP address is not flagged as a spam source. They say the email is simply not arriving.

Does anyone have any ideas why we can't send email through Comcast's SMTP server to my domains?

As an odd coincidence, we recently noticed a change in Comcast's SMTP service. there is now a 5 minute delay on all outbound mail. Comcast's SMTP server seems to sit on the mail with a 5 minute timer.