[danielweber]

I think you and the parent are using different meanings of "guarantee."

Yes, everything in the protocol leads to the fact that nothing should drop an email unless it has passed responsibility of it to another server which has accepted the message.

In practice, lots of times things don't work.

[ams6110]

Yes, one misconfigured server between source and destination can "eat" email and you'll never be the wiser. Doesn't happen often, but it can and the protocol does not detect it.