I sent an email from mike@my.dom to my_distro_list@myother.dom

One of the recipients on the my_distro_list has a server which apparently performs this "sender verify callout" which is another name for a callback, that is a reverse check to make sure the sending server -ALSO- accepts email for the original sender (i.e., mike@my.dom).

The "fix" was to make the outgoing server for myother.dom act as backup MX for my.dom

That way, when the recipient server gets the email I sent to my_distro_list, it can verify that the sending server for myother.dom also accepts email for my.dom

Presumably this check/verify helps thwart spammers, but not clear how -- if some stupid server acts as an open relay then it acts as an open relay and in so doing answers affirmative, that it does accept email for whatever initial sender domain(s).

So this whole business seems right now like a giant waste of time and it probably is, as others have asserted, a mechanism for DoS, as someone could basically make a server performing these silly "sender verify callouts" do extraneous work along with whatever unwitting target server(s) respond to those callouts.