Blocked Public Static IP Address - Comcast and CenturyLink

I am documenting an extended problem I experienced with my email in the hope that it may not occur again and that others may benefit from my experience.
I support a network with around 40 users and maybe 100 devices using Comcast Business as the ISP. My email stopped working last Sunday (7 days ago). It has now self-corrected. My email was OK at other locations using other ISP’s.My email is hosted by a company aplus.net and has been for many years. The other users on my network do not use this hosting service. Additionally I could not access www.aplus.net from browsers on computers within my network but could at the other locations. The many other users on my network did not report any problems. It seems only my email service had a problem.
My network has a static IP address on the public internet. This is provided to us by Comcast – our ISP. This became listed on the Composite Blocking List https://www.abuseat.org/lookup.cgi . I do not know why but it can happen rather easily. I successfully delisted at the second attempt on Wednesday.
A “tracert” demonstrated that Comcast passes traffic heading to aplus.net over to Level3. After that there is no response from any additional hops. The same behavior is observed now, even though the email is working. The Comcast agents and I assumed that Level3 was blocking the IP address although we have no definitive evidence.
I made many phone calls to Comcast and to Level3 (now CenturyLink) in an attempt to find out where the blocking occurred. I was frustrated in that Comcast would not talk directly to Level3 nor Level3 to Comcast. Level3 would not conduct an investigation without a trouble ticket being raised by Comcast. Since I am not a direct customer they would not create one on my behalf. Comcast would not contact Level3 to raise a ticket. They took the position that since the blocking did not occur on their network it was not their problem. Last night I discovered that Comcast and Level3 had a poor business relationship in the past relating to their “peering agreement”. Level3 provided CDN services to Netflix for a fee. Comcast felt that they should be compensated for relaying this Netflix traffic. I may have suffered from this negative history.
The problem appears to have self-corrected. I will be making fewer visits to my gym and church now that my email works again from home.
I hope that Comcast and CenturyLink will decide to work cooperatively on behalf of customers experiencing similar difficulties.