fix: run blocking SMTP calls in a thread to avoid blocking the event loop (#3988)

lnbits/core/services/notifications.py

@@ -222,12 +222,20 @@ async def send_email(
     msg["Subject"] = subject
     msg.attach(MIMEText(message, "plain"))
     username = username if len(username) > 0 else from_email
-    with smtplib.SMTP(server, port) as smtp_server:
-        smtp_server.starttls()
-        smtp_server.login(username, password)
-        smtp_server.sendmail(from_email, to_emails, msg.as_string())
+
+    def _send() -> bool:
+        with smtplib.SMTP(server, port) as smtp_server:
+            smtp_server.starttls()
+            smtp_server.login(username, password)
+            smtp_server.sendmail(from_email, to_emails, msg.as_string())
         return True
 
+    try:
+        return await asyncio.to_thread(_send)
+    except Exception as e:
+        logger.warning(f"Sending Email failed. {e!s}")
+        return False
+
 
 async def dispatch_webhook(payment: Payment):
     """