Comment by pelzatessa

It is possible for them to say that they deleted the messages without actually deleting them though. One has to trust a pretty big company in order to not worry about the messages actually not being stored anywhere.

I'm not aware of all techniques that Signal uses to somehow make the message anonymous even when if the encryption would have been broken, but sealed sender seems to be one of them:

https://signal.org/blog/sealed-sender/

So at least there's that. Unless the encrypted sealed sender messages aren't somehow being fingerprinted by the IP address of client and the timestamps of connections. Signal probably also says that they don't log these, but with self hosted mailserver I wouldn't have to trust them on that too.