I try to design a client application for a messaging application. The client can send and receive messages, the client can connect/disconnect.

The user can perform the following actions:

• Connect & Disconnect
• Display a list of currently connected users
• Send a new message
• Display current messages in a list

I have finally come up with the following UML class diagram:

The client application communicates with a server application. Every communication with the Server happens through the ConnectionManager. getReceptionTime()returns a string indicating a timestamp when the Server received the actual message. connect() sends a Message object to the Server.

My problem is that I don't know how to incorporate the Server class. Where would one put it in the diagram?

EDIT: Message class,

getSender() - returns a User object. 
setSender(User) - sets the User as sender of the Message.
getReceptionTime() - returns a timestamp when the message was delivered to the Server. 
getTimeOfDelivery() - returns a timestamp when the message was delivered to the user.
getContents() - returns an object, this object includes the message content.

Sequence diagram: THE USER CONNECTS THE CLIENT TO THE SERVER