diff --git a/backend/.env.example b/backend/.env.example
index f420a06..277157a 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -15,7 +15,11 @@
 # Esta URI debe estar registrada también en la consola de Google Cloud.
 GOOGLE_CLIENT_ID=tu-client-id.apps.googleusercontent.com
 GOOGLE_CLIENT_SECRET=tu-client-secret
-GOOGLE_REDIRECT_URI=http://localhost:8070/api/auth/google/callback
+# URI a la que Google redirige tras el login (debe estar registrada en Google Cloud)
+# Con Docker: suele ser el frontend (8071) porque /api hace proxy al backend
+GOOGLE_REDIRECT_URI=http://localhost:8071/api/auth/google/callback
+# Producción (añade la misma URI en Google Console → Credenciales OAuth):
+# GOOGLE_REDIRECT_URI=http://tu-servidor:8071/api/auth/google/callback
 
 # Clave para firmar los JWT (usa algo largo y aleatorio en producción)
 SECRET_KEY=cambia-esta-clave-en-produccion
@@ -27,16 +31,41 @@ SECRET_KEY=cambia-esta-clave-en-produccion
 FRONTEND_URL=http://localhost:5173
 
 # Verificación de email por código numérico
-# Si SMTP_HOST no está configurado, el backend imprimirá el código en consola.
+# El destinatario puede ser cualquier proveedor (Gmail, Hotmail, Outlook, etc.).
+# El fallo de envío suele deberse a SMTP mal configurado, no al dominio del usuario.
 EMAIL_VERIFICATION_CODE_TTL_MINUTES=15
 EMAIL_VERIFICATION_MAX_ATTEMPTS=5
 EMAIL_VERIFICATION_SECRET=cambia-este-secreto-en-produccion
 
-# SMTP para enviar los códigos de verificación
+# Solo desarrollo local: imprime el código en logs si SMTP falla o no existe
+# EMAIL_VERIFICATION_DEV_LOG_CODE=true
+# Muestra el error SMTP real en la respuesta HTTP (no usar en producción)
+# EMAIL_VERIFICATION_SHOW_ERRORS=true
+
+# SMTP para enviar los códigos de verificación (cuenta REMITENTE, no la del usuario)
+# Gmail: crea una "Contraseña de aplicación" en https://myaccount.google.com/apppasswords
+# SMTP_HOST=smtp.gmail.com
+# SMTP_PORT=587
+# SMTP_USE_TLS=true
+# SMTP_USE_SSL=false
+# SMTP_USERNAME=tu-correo@gmail.com
+# SMTP_PASSWORD=xxxx-xxxx-xxxx-xxxx
+# SMTP_FROM_EMAIL=tu-correo@gmail.com
+
+# Outlook / Hotmail / Microsoft 365 (cuenta remitente @outlook.com, @hotmail.com, etc.)
+# SMTP_HOST=smtp-mail.outlook.com
+# SMTP_PORT=587
+# SMTP_USE_TLS=true
+# SMTP_USE_SSL=false
+# SMTP_USERNAME=tu-correo@hotmail.com
+# SMTP_PASSWORD=tu-contraseña-o-app-password
+# SMTP_FROM_EMAIL=tu-correo@hotmail.com
+
 SMTP_HOST=smtp.gmail.com
 SMTP_PORT=587
 SMTP_USE_TLS=true
 SMTP_USE_SSL=false
+SMTP_TIMEOUT_SECONDS=30
 SMTP_USERNAME=tu-correo@gmail.com
 SMTP_PASSWORD=tu-password-o-app-password
 SMTP_FROM_EMAIL=tu-correo@gmail.com
diff --git a/backend/api/routers/auth.py b/backend/api/routers/auth.py
index f05aa5d..34e0ba8 100644
--- a/backend/api/routers/auth.py
+++ b/backend/api/routers/auth.py
@@ -1,3 +1,5 @@
+import logging
+import os
 from datetime import datetime
 
 from bson import ObjectId
@@ -10,7 +12,11 @@ from api.models.user_models import (
     UserCreate,
     UserLogin,
 )
-from api.services.email_service import send_verification_email
+from api.services.email_service import (
+    EmailConfigurationError,
+    EmailDeliveryError,
+    send_verification_email,
+)
 from api.utils.email_verification import (
     MAX_EMAIL_VERIFICATION_ATTEMPTS,
     generate_verification_code,
@@ -27,6 +33,40 @@ from api.utils.security import (
 )
 
 router = APIRouter(prefix="/auth", tags=["auth"])
+logger = logging.getLogger(__name__)
+
+
+def _env_bool(name: str, default: str = "false") -> bool:
+    return os.getenv(name, default).strip().lower() in {"1", "true", "yes", "on"}
+
+
+def _email_send_error_detail(exc: Exception) -> str:
+    if _env_bool("EMAIL_VERIFICATION_SHOW_ERRORS"):
+        return f"No se pudo enviar el correo de verificación: {exc}"
+    return "No se pudo enviar el correo de verificación"
+
+
+async def _dispatch_verification_email(email: str, username: str, code: str) -> None:
+    try:
+        sent = send_verification_email(email, username, code)
+    except (EmailConfigurationError, EmailDeliveryError) as exc:
+        logger.error("Fallo al enviar verificación a %s: %s", email, exc)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=_email_send_error_detail(exc),
+        ) from exc
+    except Exception as exc:
+        logger.exception("Error inesperado al enviar verificación a %s", email)
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=_email_send_error_detail(exc),
+        ) from exc
+
+    if not sent:
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail="No se pudo enviar el correo de verificación",
+        )
 
 
 @router.post("/register")
@@ -68,13 +108,10 @@ async def register_user(user: UserCreate):
     result = await users_collection.insert_one(user_doc)
 
     try:
-        send_verification_email(email, user.username, verification_code)
-    except Exception as exc:
+        await _dispatch_verification_email(email, user.username, verification_code)
+    except HTTPException:
         await users_collection.delete_one({"_id": result.inserted_id})
-        raise HTTPException(
-            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
-            detail="No se pudo enviar el correo de verificación",
-        ) from exc
+        raise
 
     return {
         "message": "Usuario registrado. Revisa tu correo para verificar la cuenta.",
@@ -214,13 +251,7 @@ async def resend_verification_email(payload: EmailVerificationResendRequest):
         },
     )
 
-    try:
-        send_verification_email(email, user["username"], verification_code)
-    except Exception as exc:
-        raise HTTPException(
-            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
-            detail="No se pudo enviar el correo de verificación",
-        ) from exc
+    await _dispatch_verification_email(email, user["username"], verification_code)
 
     return {"message": "Nuevo código de verificación enviado"}
 
diff --git a/backend/api/routers/google_auth.py b/backend/api/routers/google_auth.py
index c168acb..316cdfc 100644
--- a/backend/api/routers/google_auth.py
+++ b/backend/api/routers/google_auth.py
@@ -1,9 +1,11 @@
+from datetime import datetime, timedelta
+from urllib.parse import quote
+
+import httpx
+import jwt
+import os
 from fastapi import APIRouter, HTTPException, Request
 from fastapi.responses import RedirectResponse
-from datetime import datetime, timedelta
-import httpx
-import os
-import jwt
 
 from api.database.mongodb import users_collection
 
@@ -17,11 +19,18 @@ FRONTEND_URL = os.getenv("FRONTEND_URL", "http://localhost:5173")
 
 @router.get("/login")
 async def google_login():
+    if not GOOGLE_CLIENT_ID or not REDIRECT_URI:
+        raise HTTPException(
+            status_code=500,
+            detail="GOOGLE_CLIENT_ID o GOOGLE_REDIRECT_URI no configurados",
+        )
+
+    redirect_uri = quote(REDIRECT_URI, safe="")
     google_auth_url = (
         "https://accounts.google.com/o/oauth2/auth"
         "?response_type=code"
         f"&client_id={GOOGLE_CLIENT_ID}"
-        f"&redirect_uri={REDIRECT_URI}"
+        f"&redirect_uri={redirect_uri}"
         "&scope=openid%20email%20profile"
         "&access_type=offline"
         "&prompt=consent"
diff --git a/backend/api/services/email_service.py b/backend/api/services/email_service.py
index 8976813..86bca83 100644
--- a/backend/api/services/email_service.py
+++ b/backend/api/services/email_service.py
@@ -1,12 +1,24 @@
 import html
+import logging
 import os
 import smtplib
 import ssl
 from datetime import datetime
 from email.message import EmailMessage
+from email.utils import formataddr
 
 from api.utils.email_verification import EMAIL_VERIFICATION_CODE_TTL_MINUTES
 
+logger = logging.getLogger(__name__)
+
+
+class EmailConfigurationError(RuntimeError):
+    """SMTP no está configurado o falta información obligatoria."""
+
+
+class EmailDeliveryError(RuntimeError):
+    """El servidor SMTP rechazó el envío o no se pudo conectar."""
+
 
 def _env_bool(name: str, default: str = "false") -> bool:
     return os.getenv(name, default).strip().lower() in {"1", "true", "yes", "on"}
@@ -142,46 +154,142 @@ def _build_verification_html(username: str, code: str) -> str:
 </html>"""
 
 
-def send_verification_email(recipient_email: str, username: str, code: str) -> bool:
-    smtp_host = os.getenv("SMTP_HOST")
-    if not smtp_host:
-        print(
-            "[email-verification] SMTP_HOST no configurado. "
-            f"Código para {recipient_email}: {code}"
+def _log_verification_code_for_dev(recipient_email: str, code: str) -> None:
+    logger.warning(
+        "[email-verification] Modo desarrollo: código para %s → %s",
+        recipient_email,
+        code,
+    )
+
+
+def _build_verification_message(
+    recipient_email: str,
+    username: str,
+    code: str,
+    smtp_from_email: str,
+    smtp_from_name: str,
+) -> EmailMessage:
+    message = EmailMessage()
+    message["Subject"] = "Codigo de verificacion - Deck of Cards"
+    message["From"] = formataddr((smtp_from_name, smtp_from_email))
+    message["To"] = recipient_email
+    message["Date"] = datetime.utcnow().strftime("%a, %d %b %Y %H:%M:%S +0000")
+    message.set_content(_build_verification_plain(username, code))
+    message.add_alternative(_build_verification_html(username, code), subtype="html")
+    return message
+
+
+def _send_via_smtp(
+    message: EmailMessage,
+    recipient_email: str,
+    smtp_from_email: str,
+    smtp_host: str,
+    smtp_port: int,
+    smtp_username: str | None,
+    smtp_password: str | None,
+    use_tls: bool,
+    use_ssl: bool,
+) -> None:
+    context = ssl.create_default_context()
+    timeout = int(os.getenv("SMTP_TIMEOUT_SECONDS", "30"))
+    debug_level = 1 if _env_bool("SMTP_DEBUG") else 0
+
+    try:
+        if use_ssl:
+            server = smtplib.SMTP_SSL(
+                smtp_host,
+                smtp_port,
+                timeout=timeout,
+                context=context,
+            )
+        else:
+            server = smtplib.SMTP(smtp_host, smtp_port, timeout=timeout)
+
+        with server:
+            server.set_debuglevel(debug_level)
+            server.ehlo()
+            if use_tls and not use_ssl:
+                server.starttls(context=context)
+                server.ehlo()
+            if smtp_username and smtp_password:
+                server.login(smtp_username, smtp_password)
+            server.sendmail(
+                smtp_from_email,
+                [recipient_email],
+                message.as_string(),
+            )
+    except smtplib.SMTPException as exc:
+        logger.exception(
+            "Error SMTP al enviar verificación a %s via %s:%s",
+            recipient_email,
+            smtp_host,
+            smtp_port,
+        )
+        raise EmailDeliveryError(str(exc)) from exc
+    except OSError as exc:
+        logger.exception(
+            "No se pudo conectar al servidor SMTP %s:%s",
+            smtp_host,
+            smtp_port,
+        )
+        raise EmailDeliveryError(str(exc)) from exc
+
+
+def send_verification_email(recipient_email: str, username: str, code: str) -> bool:
+    recipient_email = recipient_email.strip().lower()
+    smtp_host = os.getenv("SMTP_HOST", "").strip()
+    dev_log_code = _env_bool("EMAIL_VERIFICATION_DEV_LOG_CODE")
+
+    if not smtp_host:
+        if dev_log_code:
+            _log_verification_code_for_dev(recipient_email, code)
+            return True
+        logger.error(
+            "[email-verification] SMTP_HOST no configurado; no se envió correo a %s",
+            recipient_email,
+        )
+        raise EmailConfigurationError(
+            "SMTP_HOST no está configurado en el servidor"
         )
-        return False
 
     smtp_port = int(os.getenv("SMTP_PORT", "587"))
-    smtp_username = os.getenv("SMTP_USERNAME")
-    smtp_password = os.getenv("SMTP_PASSWORD")
-    smtp_from_email = os.getenv("SMTP_FROM_EMAIL") or smtp_username
+    smtp_username = (os.getenv("SMTP_USERNAME") or "").strip()
+    smtp_password = (os.getenv("SMTP_PASSWORD") or "").replace(" ", "")
+    smtp_from_email = (os.getenv("SMTP_FROM_EMAIL") or smtp_username or "").strip()
     smtp_from_name = os.getenv("SMTP_FROM_NAME", "Deck of Cards")
     use_tls = _env_bool("SMTP_USE_TLS", "true")
     use_ssl = _env_bool("SMTP_USE_SSL", "false")
 
     if not smtp_from_email:
-        raise RuntimeError("SMTP_FROM_EMAIL o SMTP_USERNAME debe estar configurado")
+        raise EmailConfigurationError(
+            "SMTP_FROM_EMAIL o SMTP_USERNAME debe estar configurado"
+        )
 
-    message = EmailMessage()
-    message["Subject"] = "Código de verificación — Deck of Cards"
-    message["From"] = f"{smtp_from_name} <{smtp_from_email}>"
-    message["To"] = recipient_email
-    message.set_content(_build_verification_plain(username, code))
-    message.add_alternative(_build_verification_html(username, code), subtype="html")
+    message = _build_verification_message(
+        recipient_email,
+        username,
+        code,
+        smtp_from_email,
+        smtp_from_name,
+    )
 
-    context = ssl.create_default_context()
-
-    if use_ssl:
-        with smtplib.SMTP_SSL(smtp_host, smtp_port, context=context) as server:
-            if smtp_username and smtp_password:
-                server.login(smtp_username, smtp_password)
-            server.send_message(message)
-    else:
-        with smtplib.SMTP(smtp_host, smtp_port) as server:
-            if use_tls:
-                server.starttls(context=context)
-            if smtp_username and smtp_password:
-                server.login(smtp_username, smtp_password)
-            server.send_message(message)
+    try:
+        _send_via_smtp(
+            message,
+            recipient_email,
+            smtp_from_email,
+            smtp_host,
+            smtp_port,
+            smtp_username,
+            smtp_password,
+            use_tls,
+            use_ssl,
+        )
+    except EmailDeliveryError:
+        if dev_log_code:
+            _log_verification_code_for_dev(recipient_email, code)
+            return True
+        raise
 
+    logger.info("Correo de verificación enviado a %s", recipient_email)
     return True