no, there is no important metadata leaked via headers, the only obvious data the server knows is the same as on Matrix and XMPP, when you tell the server to deliver the message to a given user, the server hence knows that X sent a message to Y, but since accounts are randomly generated email addresses without any personal data tied, this is more secure than Signal where phone numbers are collected by a central server and if they wanted to they can match your IP to your phone number even if you use sealed sender to send it is obvious who you are by your IP and timing since you also connect to the server non-anonymously to receive messages