diff --git a/src/authd.cr b/src/authd.cr
index b7c544e..c15c070 100644
--- a/src/authd.cr
+++ b/src/authd.cr
@@ -39,5 +39,8 @@ require "./authd/exceptions"
 # Requests and responses.
 require "./network"
 
+# Run child processes.
+require "./process"
+
 # Functions to request the authd server.
 require "./authd/client.cr"
diff --git a/src/authd/user.cr b/src/authd/user.cr
index 7bafbf0..552f53b 100644
--- a/src/authd/user.cr
+++ b/src/authd/user.cr
@@ -26,6 +26,10 @@ class AuthD::User
 		property activation_key : String? = nil
 		property email          : String?
 
+		# Not yet validated email address: useful to keep a the previous validated email address
+		# until the new address is validated.
+		property pending_email  : String? = nil
+
 		def initialize(@email = nil)
 		end
 
diff --git a/src/process.cr b/src/process.cr
new file mode 100644
index 0000000..36655fe
--- /dev/null
+++ b/src/process.cr
@@ -0,0 +1,24 @@
+def run_process(cmd : String, params : Array(String), env : Hash(String, String))
+	unless Process.run(cmd, params, env,
+			true # clear environment
+			# input:   Process::Redirect::Inherit,
+			# output:  Process::Redirect::Inherit,
+			# error:   Process::Redirect::Inherit
+		).success?
+		raise "cannot run #{cmd} #{params.join(" ")}"
+	end
+end
+
+# Send a token to recovery the user's password.
+def send_recovery_token(authd : AuthD::Service, login : String, email : String, token : String)
+	run_process(authd.configuration.mailer_exe,
+		[ "send", authd.configuration.recovery_template, email ],
+		{ "HOME" => "/", "LOGIN" => login, "TOKEN" => token })
+end
+
+# Send a token to validate the user's email address.
+def send_activation_token(authd : AuthD::Service, login : String, email : String, token : String)
+	run_process(authd.configuration.mailer_exe,
+		[ "send", authd.configuration.activation_template, email ],
+		{ "HOME" => "/", "LOGIN" => login, "TOKEN" => token })
+end
diff --git a/src/requests/moduser.cr b/src/requests/moduser.cr
index 92962b0..1827ceb 100644
--- a/src/requests/moduser.cr
+++ b/src/requests/moduser.cr
@@ -33,8 +33,28 @@ class AuthD::Request
 				cloned_user.password_hash = authd.hash_password s
 			end
 
-			@email.try do |email|
-				cloned_user.contact.email = email
+			# In case of a new email address:
+			# 1. the new address is stored as "pending_email"
+			# 2. the new address has to be validated before being used as primary email address
+			if email = @email
+				# Verify the email address isn't already in the database.
+				if authd.users_per_email.get? Base64.encode(email).chomp
+					return Response::ErrorEmailAddressAlreadyUsed.new
+				end
+				cloned_user.contact.pending_email = email
+				cloned_user.contact.new_activation_key
+
+				begin
+					u_login            = cloned_user.login
+					u_email            = cloned_user.contact.pending_email.not_nil!
+					u_activation_token = cloned_user.contact.activation_key.not_nil!
+
+					# Once the user is created and stored, we try to contact him.
+					send_activation_token authd, u_login, u_email, u_activation_token
+				rescue e
+					Baguette::Log.error "mailer: #{e}"
+					return Response::ErrorCannotContactUser.new
+				end
 			end
 
 			begin
diff --git a/src/requests/password.cr b/src/requests/password.cr
index dea0c30..cf5b7aa 100644
--- a/src/requests/password.cr
+++ b/src/requests/password.cr
@@ -35,26 +35,11 @@ class AuthD::Request
 					user.password_renew_key.not_nil!
 			end
 
-			mailer_exe = authd.configuration.mailer_exe
-			template_name = authd.configuration.recovery_template
-
 			u_login = user.login
 			u_email = user.contact.email.not_nil!
 			u_token = user.password_renew_key.not_nil!
 
-			# Once the user is created and stored, we try to contact him.
-			unless Process.run(mailer_exe,
-					# PARAMETERS
-					[ "send", template_name, u_email ],
-					# ENV
-					{ "HOME" => "/", "LOGIN" => u_login, "TOKEN" => u_token },
-					true # clear environment
-					# input:   Process::Redirect::Inherit,
-					# output:  Process::Redirect::Inherit,
-					# error:   Process::Redirect::Inherit
-				).success?
-				raise "cannot contact user #{u_login} address #{u_email}"
-			end
+			send_recovery_token authd, u_login, u_email, u_token
 
 			Response::PasswordRecoverySent.new
 		end
diff --git a/src/requests/register.cr b/src/requests/register.cr
index 4cbd90b..ef6e8e2 100644
--- a/src/requests/register.cr
+++ b/src/requests/register.cr
@@ -25,13 +25,16 @@ class AuthD::Request
 				return Response::ErrorMailRequired.new
 			end
 
-			if ! @email.nil?
+			if m = @email
 				# Test on the email address format.
 				grok = Grok.new [ "%{EMAILADDRESS:email}" ]
-				result = grok.parse @email.not_nil!
+				result = grok.parse m
 				email = result["email"]?
 
 				return Response::ErrorInvalidEmailFormat.new if email.nil?
+
+				# Verify the email address isn't already in the database.
+				return Response::ErrorEmailAddressAlreadyUsed.new if authd.users_per_email.get? Base64.encode(m).chomp
 			end
 
 			# In this case we should not accept its registration.
@@ -42,7 +45,7 @@ class AuthD::Request
 			password = authd.hash_password @password
 
 			user = User.new uid, @login, password
-			user.contact.email = @email unless @email.nil?
+			user.contact.pending_email = @email unless @email.nil?
 			user.contact.new_activation_key
 
 			@profile.try do |profile|
@@ -52,26 +55,12 @@ class AuthD::Request
 			user.date_registration = Time.local
 
 			begin
-				mailer_exe       = authd.configuration.mailer_exe
-				template_name    = authd.configuration.activation_template
-
-				u_login          = user.login
-				u_email          = user.contact.email.not_nil!
-				u_activation_key = user.contact.activation_key.not_nil!
+				u_login            = user.login
+				u_email            = user.contact.pending_email.not_nil!
+				u_activation_token = user.contact.activation_key.not_nil!
 
 				# Once the user is created and stored, we try to contact him.
-				unless Process.run(mailer_exe,
-						# PARAMETERS
-						[ "send", template_name, u_email ],
-						# ENV
-						{ "HOME" => "/", "LOGIN" => u_login, "TOKEN" => u_activation_key },
-						true # clear environment
-						# input:   Process::Redirect::Inherit,
-						# output:  Process::Redirect::Inherit,
-						# error:   Process::Redirect::Inherit
-					).success?
-					raise "cannot contact user #{u_login} address #{u_email}"
-				end
+				send_activation_token authd, u_login, u_email, u_activation_token
 			rescue e
 				Baguette::Log.error "mailer: #{e}"
 				return Response::ErrorCannotContactUser.new
diff --git a/src/requests/users.cr b/src/requests/users.cr
index 83bc00e..a5e5e29 100644
--- a/src/requests/users.cr
+++ b/src/requests/users.cr
@@ -17,13 +17,17 @@ class AuthD::Request
 			end
 
 			# Remove the user contact activation key: the email is validated.
-			if user.contact.activation_key == @activation_key
-				user.contact.activation_key = nil
-			else
+			if user.contact.activation_key != @activation_key
 				return Response::ErrorInvalidActivationKey.new
 			end
 
-			authd.users_per_uid.update user.uid.to_s, user
+			cloned_user = user.clone
+
+			cloned_user.contact.activation_key = nil
+			cloned_user.contact.email = cloned_user.contact.pending_email
+			cloned_user.contact.pending_email = nil
+
+			authd.users_per_uid.update cloned_user
 
 			Response::UserValidated.new user.to_public
 		end
diff --git a/src/responses/errors.cr b/src/responses/errors.cr
index 81d34a2..ecb5886 100644
--- a/src/responses/errors.cr
+++ b/src/responses/errors.cr
@@ -108,4 +108,10 @@ class AuthD::Response
 		end
 	end
 	AuthD.responses << ErrorEmailAddressNotValidated
+
+	IPC::JSON.message ErrorEmailAddressAlreadyUsed, 37 do
+		def initialize()
+		end
+	end
+	AuthD.responses << ErrorEmailAddressAlreadyUsed
 end