Hi @philpav , @PoJo
The challenge that exists here, is that not all mail servers will return a bounce message when an email is undelivered. Most will, in most cases, but not all. This is particularly the case with spam filters.
It is therefore not guaranteed that a sender (or a sender's system) will always know if a sent message has been undeliverable to its final destination.
Bad email addresses will almost always get bounced. But spam filters can often be configured to discard or quarantine email messages totally silently (without informing the sender).
Spam filters can be configured entirely at the discretion of the recipient's organization. Almost all end up with different rules and levels of sensitivity.
With quarantining, usually the recipient, or an intermediate system administrator, would be informed on, typically, a periodic basis - but this relies on them having a keen eye on their reports. The sender, however, or a sender's system, would usually have no way of knowing their message was quarantined (no bounce for quarantine).
If a spam filter has been configured to silently discard what it believes to be spam of a certain type, then there is simply no way to know. It may not be desirable, or pleasant, but it can happen.
Usually it is the responsibility of the recipient's organization to ensure they can receive email from their desired senders - patricianly when it comes to spam filtering.
If meduniwien.ac.at has a particular problem receiving email from Zoom, I would be leaning on them to fix that.