have to? no, absolutely
can reasonably choose to do for defensible reasons? sure!
eventual consistency is a lot more efficient than globally synchronous lockstep operation in distributed systems
there was a time when SMTP servers hadn't got so centralized, and policies and decisions may have been made at a time in which such delays were an inherent part of the system, and people cared and complained about such things
the fact that now people lacking (or forgetting) that context expect instant reactions doesn't justify rebuilding systems designed and implemented based on earlier assumptions that remain reasonable
and unless you're paying for their angry (former) subscriber support, maybe you could give them a little more credit for trying to set reasonable expectations, and not assign to malice what can be explained by (others') stupidity ;-)