Email delivery is not a guaranteed process.  It does NOT depend upon one set of computer / router configurations.  It does NOT depend upon one set of telecommunication circuits.  It does NOT depend upon one fixed-set of personnel.  It is forever changing.  CHANGE is accomplished by fallible people.

Futhermore, LISTSERV(R) adds a level of complexity over-on-top of those processes.

I can have multiple subscribers from the exact same email address: some will get the distributions and some will not, on occasion.

Even when I know what I think the problem is, finding and working with competent gurus is also a challenge (sometimes).