That's not true. It should either send the email to the recipient or return it to sender. But it should never just drop the message (except for bounces which can be dropped since there is nowhere to send them if delivery fails).

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.

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.

An SMTP daemon will try to send the message a certain number of times, at a certain interval, then, eventually, it will stop ("bounce"). You can configure these settings if you run your own SMTP service.