email server currently has 140 errors like this for the last 24 hours
dsn=4.0.0, stat=Deferred: xxx: No route to host
I call that an invalid email address.
Yes it's a static IP, yes it pings, but no there's no email server there.
That is an email
server problem, not an email
address problem. An email address problem would result in a rejection, not a deferral.
Same thing.