diff --git a/MailSystem/src/domain/EasyMail.java b/MailSystem/src/domain/EasyMail.java
index 2b9278b..c22a037 100644
--- a/MailSystem/src/domain/EasyMail.java
+++ b/MailSystem/src/domain/EasyMail.java
@@ -21,8 +21,8 @@ public class EasyMail {
 			}
 	}
 
-	public void userRegister(String firstname, String lastName, String username, int year, int day, String monthName,char[] password, char[] passwordConfirmation) throws Exception {
-		this.currentUser = userManager.addUser(firstname, lastName, username, year, day, monthName, password,passwordConfirmation);
+	public void userRegister(String firstname, String lastName, String email, int year, int day, String monthName,char[] password, char[] passwordConfirmation) throws Exception {
+		this.currentUser = userManager.addUser(firstname, lastName, email, year, day, monthName, password,passwordConfirmation);
 	}
 
 	public boolean userSignIn(String username, char[] password) throws Exception {
@@ -73,7 +73,7 @@ public class EasyMail {
 	public String[] sendUserDetails() {
 		String[] details = new String[2];
 		String name = this.currentUser.getFirstname() + " " + this.currentUser.getLastname();
-		String username =  this.currentUser.getUsermail().getUsername();
+		String username =  this.currentUser.getUsermail().getUserEmail();
 		details[0] = name; 
 		details[1] = username;
 		
@@ -81,7 +81,7 @@ public class EasyMail {
 	}
 	
 	public String getUsernameFromCurrentUser() {
-		return this.currentUser.getUsermail().getUsername();
+		return this.currentUser.getUsermail().getUserEmail();
 	}
 
 	
diff --git a/MailSystem/src/domain/email/Email.java b/MailSystem/src/domain/email/Email.java
index 94ba5b6..ed837b3 100644
--- a/MailSystem/src/domain/email/Email.java
+++ b/MailSystem/src/domain/email/Email.java
@@ -42,10 +42,10 @@ public class Email {
 	}
 
 	public String showEmailsInSent() {
-		return receiver.getUsermail().getUsername() + "," +  subject + "," + formattDate() + "," + content ;
+		return receiver.getUsermail().getUserEmail() + "," +  subject + "," + formattDate() + "," + content ;
 	}
 	
 	public String showEmails() {
-		return sender.getUsermail().getUsername() + "," +  subject + "," + formattDate() + "," + content ;
+		return sender.getUsermail().getUserEmail() + "," +  subject + "," + formattDate() + "," + content ;
 	}
 }
diff --git a/MailSystem/src/domain/user/User.java b/MailSystem/src/domain/user/User.java
index 0dba855..37a91a2 100644
--- a/MailSystem/src/domain/user/User.java
+++ b/MailSystem/src/domain/user/User.java
@@ -9,14 +9,14 @@ public class User {
 	private String firstname;
 	private String lastname;
 	private LocalDate birthdate;
-	private UserEmail usermail;
+	private UserEmail userEmail;
 
-	public User(String firstname, String lastname, LocalDate birthdate,String nutzername, char[] password) {
+	public User(String firstname, String lastname, LocalDate birthdate,String email, char[] password) {
 		this.userID = counter++;
 		this.firstname = firstname;
 		this.lastname = lastname;
 		this.birthdate = birthdate;
-		this.usermail = new UserEmail(nutzername,password);
+		this.userEmail = new UserEmail(email,password);
 	}
 
 	public String getFirstname() {
@@ -48,7 +48,7 @@ public class User {
 	}
 
 	public UserEmail getUsermail() {
-		return usermail;
+		return userEmail;
 	}
 
 }
diff --git a/MailSystem/src/domain/user/UserEmail.java b/MailSystem/src/domain/user/UserEmail.java
index ead405d..29b07cc 100644
--- a/MailSystem/src/domain/user/UserEmail.java
+++ b/MailSystem/src/domain/user/UserEmail.java
@@ -6,7 +6,7 @@ public class UserEmail {
 
 	private static int counter = 1000;
 	private int accountID;
-	private String username;
+	private String email;
 	private boolean status;
 	private char[] password;
 	private SentFolder sentFolder;
@@ -14,7 +14,7 @@ public class UserEmail {
 	private Inbox inbox;
 	
 	public UserEmail(String username, char[] password) {
-		this.username = username;
+		this.email = username;
 		this.password = password;
 		this.accountID = counter++;
 		this.status = true;
@@ -23,8 +23,8 @@ public class UserEmail {
 		this.inbox = new Inbox();
 	}
 
-	public String getUsername() {
-		return username;
+	public String getUserEmail() {
+		return email;
 	}
 
 	public  char[] getPassword() {
diff --git a/MailSystem/src/domain/user/UserManager.java b/MailSystem/src/domain/user/UserManager.java
index ab1257f..e945591 100644
--- a/MailSystem/src/domain/user/UserManager.java
+++ b/MailSystem/src/domain/user/UserManager.java
@@ -3,21 +3,20 @@ package domain.user;
 import java.time.LocalDate;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.HashMap;
 
 
 public class UserManager {
 
-    private ArrayList<User> users;
+    private HashMap<String,User> users;
     
     public UserManager(){
-        this.users = new ArrayList<>();
-
-       
+        this.users = new HashMap<>();
     }
-	public User addUser(String firstName, String lastName, String username, int year, int day, String monthName,
+	public User addUser(String firstName, String lastName, String email, int year, int day, String monthName,
 			char[] password, char[] passwordConfirmation) throws Exception {
 
-		if (firstName.trim().isEmpty() || lastName.trim().isEmpty() || username.trim().isEmpty() || password.length == 0
+		if (firstName.trim().isEmpty() || lastName.trim().isEmpty() || email.trim().isEmpty() || password.length == 0
 				|| passwordConfirmation.length == 0) {
 			throw new IllegalArgumentException("All fields are required!");
 		}
@@ -26,11 +25,13 @@ public class UserManager {
 		
 		if (!Arrays.equals(password, passwordConfirmation))
 			throw new IllegalArgumentException("Passwords do not match!");
-
-		String email = username + "@easymail.de";
-		if (findUserByUsername(email) != null)
-			throw new UserAlreadyExistsException("This email address is already taken!");
-
+		
+		String domain = "@easymail.de";
+		email += domain;
+		
+        if (users.containsKey(email)) 
+            throw new UserAlreadyExistsException("Email already registered!");
+        
 		int month = getMonthNumber(monthName);
 		if (month == 0)
 			throw new IllegalArgumentException("Invalid month name: " + monthName);
@@ -39,48 +40,57 @@ public class UserManager {
 		char[] passwordCopy = Arrays.copyOf(password, password.length);
 
 		User newUser = new User(firstName, lastName, birthDate, email, passwordCopy);
-
-		users.add(newUser);
-
+		users.put(email, newUser);
+		
 		Arrays.fill(password, ' ');
 		Arrays.fill(passwordConfirmation, ' ');
 		return newUser;
 	}
 
-	public User checkLogin(String username, char[] password)throws Exception{
-		if (username.trim().isEmpty() || password.length == 0)
-			throw new UserAlreadyExistsException("All fields are required!");
+	public User checkLogin(String userEmail, char[] password) throws Exception {
+      
+		if (userEmail.trim().isEmpty()|| password.length == 0) 
+			throw new IllegalArgumentException("All fields are required!");
+	
+        // Email generieren (case-insensitive)
+		userEmail = userEmail.toLowerCase();
+        User user = users.get(userEmail);
+        if (user == null) {
+            Arrays.fill(password, ' ');
+            throw new UserNotFoundException("User not found!");
+        }
 
-		for (User user : users)
-			if (user.getUsermail().getUsername().equalsIgnoreCase(username)
-					&& Arrays.equals(user.getUsermail().getPassword(), password)) {
-				Arrays.fill(password, ' ');
-				return user;
-			}
-		Arrays.fill(password, ' ');
-		throw new UserNotFoundException("This email address is not found!");
-	}
+        if (!Arrays.equals(user.getUsermail().getPassword(), password)) {
+            Arrays.fill(password, ' ');
+            throw new SecurityException("Invalid password!");
+        }
 
-	public boolean removeUser(String username) throws UserNotFoundException {
-		if (username == null || username.trim().isEmpty())
-			throw new IllegalArgumentException("Username cannot be null or empty!");
+        Arrays.fill(password, ' ');
+        return user;
+    }
 
-		User userToBeRemoved = findUserByUsername(username);
-		if (userToBeRemoved == null)
-			throw new UserNotFoundException("This email address is not found!");
 
-		users.remove(userToBeRemoved);
-		return true;
-	}
+	public boolean removeUser(String userEmail) throws UserNotFoundException {
+		if (userEmail.trim().isEmpty())
+			throw new IllegalArgumentException("email is required!");
+			
+		userEmail = userEmail.toLowerCase();
+        User removed = users.remove(userEmail);
+        
+        if (removed == null) 
+            throw new UserNotFoundException("User not found!");
+        
+        return true;
+    }
 
 	public int getNumberOfUsers() {
 		return users.size();
 	}
 
-	public User updateUser(String firstName, String lastName, String username, int year, int day, String monthName,
+	public User updateUser(String firstName, String lastName, String userEmail, int year, int day, String monthName,
 	        char[] password, char[] passwordConfirmation) throws Exception {
 
-	    User userToBeUpdated = findUserByUsername(username);
+	    User userToBeUpdated = findUserByUsername(userEmail);
 	    if (userToBeUpdated == null)
 	        throw new UserNotFoundException("This email address is not found!");
 
@@ -126,12 +136,8 @@ public class UserManager {
 
 
 	
-	public User findUserByUsername(String username) {
-		for (User tempUser : users)
-			if (tempUser.getUsermail().getUsername().equalsIgnoreCase(username))
-				return tempUser;
-
-		return null;
+	public User findUserByUsername(String userEmail) {
+        return users.get(userEmail);
 	}
 
 	private int getMonthNumber(String txtMonth) {