Although my earlier topic about the same subject and was locked out by the forum moderator as a resolved I still experience the problem.
1. The problem did not happen when I use Xfinity webmailer.
2. The problem intermittently occurs when I use MS Outlook 2007 client application and only with one known to my email recipient.
3. When the blocking of outgoing message occurs I receive email@example.com message with diagnostic - code: SMTP; 554
5.7.1 Message refused by DNSBL check. bl.spamcop.net
4. The troublesome mail server is resqmta-ch2-09v.sys.comcast.net [ 188.8.131.52]
5. The blocking happened on 12/25 at 10PM and cleared on 12PM the following day and happened again today at 12:36 PM.
6. I decided to turn on in Outlook.log allowing me to check whether there might be issues with Outlook mailer.
7. I gathered OPMLog.log file with exact timing of all processes, following each step - Outlook mailer exactly exercised all outbound
processes with the first in chain mail server resomta-ch2-07v.sys.comcast.net[184.108.40.206]
8. The conclusion is the issue is with the second Comcast mail server which is marked by SPAMCOP.NET as a spamer putting it on their blacklist.
9. Reading the required procedures: after a certain period of time the server can be released from blacklist or Comcast administrator shall fill out a special form advising SPAMCOP that their server does not cause spam.
10. Outlook mailer is not designed to have a log file of processes happened in the second mail server unless the administrator in Texas has that right of access to ( there where is located this server ).
I guess Comcast Security Assurance people sloppely do that task relying that it automatically clears up by itself.
I spoke twice to Comcast chat agents who created a case number promissing to talk to next level of technical expertise.
An exact same problem was described by author JJMcVeigh back in 10-17-2016 when Comcast could not find a solution for it either.
I received last week a phone call from Comcast that they solved the problem asking me to check whether I do not receive more Permanent Errors from firstname.lastname@example.org. So far, there have not been more problems with my email.
Comcast did not share with me how the problem was solved on their end.