Not getting USPS Informed Delivery emails

@XfinityAlfonso Thanks for your quick response. No, supposedly large organizations like the USPS use an ESP for their bulk/marketing emails. According to the thread, the USPS used to use MailGun, then switched to Sinch (not sure if that is still the case). XfinityAlex was asking people to DM him and he was able to work with Sinch to unsuspend their email address. Hope this information helps.

Thanks @XfinityAlfonso , after hours of phone calls and chats with Xfinity, I now have a case open with the Customer Security Assurance level 3 team. No resolution yet, but making slow progress.

Thanks for the advice and the education on what the acronym ESP stands for. I was not familiar with an email service provider and was not speaking to the readers of this forum but hope they can get assistance if they find themselves in the same situation. I was posting to the Xfinity support experts. 
This post from XfinityAlex several months ago is what I was referencing. He did not follow your advice.
From XfinityAlex:
In the email world, there are MBPs/ISPs (like comcast.net or gmail.com), and then there are ESPs (such as Mailgun or SendGrid).  Typically, the ESPs send a large volume of messages on behalf of brands/entities (USPS, Home Depot, LL Bean, etc), and there's a lot of orchestration that goes on behind the scenes.  A user/customer orders something, or signs up for a service, or mailing list, and then that eventually propagates over to the ESP where they manage things such as segmentation and engagement.  In many cases, when users/customers mark a message as spam (intentionally or accidentally), a report (FBL is the acronym) goes back to the ESP.  The ESP believes you no longer want to receive such messages, and then adds you to what is called a "suppression list".  And sometimes when you're on such a list, no matter how many times you unsubscribe or resubscribe, it is of no use (depending how the data is managed, etc).  The problem is that most users don't know these exist, or how to get their "suppression" removed.  The result is that we reach out to the ESP on your behalf to ask them to remove that.

Hi @voicon1806 Thank you for commenting on the thread. Based on the information provided in this thread and information we are aware of, you would need to reach out to customer security team. Please check out Customer Security Assurance.