Signal certeinly can store it, because they have access to it.
They have access to that data alone through the fact that they are in control of their servers and, thus, can see who sends and receives messages.
They have access to that data alone through the fact that they are in control of their servers and, thus, can see who sends and receives messages.
Even on a theoretical level their sealed sender technique doesn't work: https://www.ndss-symposium.org/ndss-paper/improving-signals-...
Now, include lower level technical Details such as the IP layer.
1. Imagine you connect to a server to send a message. Now, you send a message to someone else. The server can't see who you are, right? Because the letter misses your name. Imagine someone else sends a message to you.
2. Imagine you to connect to the same server to receive messages that the server stored for delivery with your name on it. The server gives you the messages.
Do you notice something?
The information can be obtained in an automated way.