diff --git a/internal/validate.go b/internal/validate.go
index da8b35cd3..c831565f5 100644
--- a/internal/validate.go
+++ b/internal/validate.go
@@ -38,9 +38,9 @@ var (
 	ErrPasswordTooLong    = fmt.Errorf("password too long: max %d characters", maxPasswordLength)
 	ErrPasswordWeak       = fmt.Errorf("password too weak: min %d characters", minPasswordLength)
 	ErrUsernameTooLong    = fmt.Errorf("username exceeds the maximum length of %d characters", maxUsernameLength)
-	ErrUsernameInvalid    = errors.New("username can only contain characters a-z, 0-9, or '_-./='")
+	ErrUsernameInvalid    = errors.New("username can only contain characters a-z, 0-9, or '_+-./='")
 	ErrUsernameUnderscore = errors.New("username cannot start with a '_'")
-	validUsernameRegex    = regexp.MustCompile(`^[0-9a-z_\-=./]+$`)
+	validUsernameRegex    = regexp.MustCompile(`^[0-9a-z_\-+=./]+$`)
 )
 
 // ValidatePassword returns an error if the password is invalid
diff --git a/internal/validate_test.go b/internal/validate_test.go
index cd2626133..1019102df 100644
--- a/internal/validate_test.go
+++ b/internal/validate_test.go
@@ -129,6 +129,11 @@ func Test_validateUsername(t *testing.T) {
 			localpart: "i_am_allowed=1",
 			domain:    "localhost",
 		},
+		{
+			name:      "special characters are allowed 3",
+			localpart: "+55555555555",
+			domain:    "localhost",
+		},
 		{
 			name:      "not all special characters are allowed",
 			localpart: "notallowed#", // contains #
@@ -139,6 +144,16 @@ func Test_validateUsername(t *testing.T) {
 				JSON: spec.InvalidUsername(ErrUsernameInvalid.Error()),
 			},
 		},
+		{
+			name:      "not all special characters are allowed 2",
+			localpart: "<notallowed", // contains <
+			domain:    "localhost",
+			wantErr:   ErrUsernameInvalid,
+			wantJSON: &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: spec.InvalidUsername(ErrUsernameInvalid.Error()),
+			},
+		},
 		{
 			name:      "username containing numbers",
 			localpart: "hello1337",