The blame game.... email issues

I'm hoping someone here has a bit of advise for me. It's late and I apologize if I'm posting this in the wrong forum.

I'm having a hard time with email from site "A" reaching site "B" (and also "C"). Site "A" is actually a paid service. We never had any problems with getting the email from "A" until just over a month ago. "A" blames the place that has "B". "A" says their logs show the emails as going out, but they are being blocked. The host for "B" says the emails are never making it to the server. I've been going back and forth on this for over a month now with both of them.

Emails from "A" seem to be able to reach any other isp in the country, except for the two domains hosted by "B". There are some days where someone at "B" will say they have changed a setting on the firewall. Oddly enough when "B" changes some sort of setting on the firewall, the emails from "A" get through for about a day or so. Even old emails that are two days old arrive. Then you have days like today where all I get is "I have tested this for you in the past. It is not a problem with our server. The email simply never reaches our machine. You will need to have this resolved by "A" "

I admit I'm not much of a technical person at all and have a lot to learn, but when the email works after a change has been made by "B" doesn't that point to a problem with "B", although they say there is no problem with their machine?

 

 

 

 

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