fix(blocks): validate email recipients in Gmail blocks before API call

Addresses #11954 — GmailSendBlock crashes with an opaque "Invalid To
header" HttpError 400 when the LLM (or user) supplies a malformed
recipient such as a bare username, a JSON string, or an empty value.

Add a lightweight `validate_email_recipients()` check in the shared
`create_mime_message()` path and in `_build_reply_message()` so that
every Gmail block that sends or drafts email gets upfront validation
with a clear, actionable error message listing the invalid entries.

autogpt_platform/backend/backend/blocks/google/gmail.py

@@ -1,5 +1,6 @@
 import asyncio
 import base64
+import re
 from abc import ABC
 from email import encoders
 from email.mime.base import MIMEBase
@@ -46,6 +47,27 @@ def serialize_email_recipients(recipients: list[str]) -> str:
     return ", ".join(recipients)
 
 
+# RFC 5322 simplified pattern: local@domain where domain has at least one dot
+_EMAIL_RE = re.compile(r"^[^@\s]+@[^@\s]+\.[^@\s]+$")
+
+
+def validate_email_recipients(
+    recipients: list[str], field_name: str = "to"
+) -> None:
+    """Validate that all recipients are plausible email addresses.
+
+    Raises ``ValueError`` with a user-friendly message listing every
+    invalid entry so the caller (or LLM) can correct them in one pass.
+    """
+    invalid = [addr for addr in recipients if not _EMAIL_RE.match(addr.strip())]
+    if invalid:
+        formatted = ", ".join(f"'{a}'" for a in invalid)
+        raise ValueError(
+            f"Invalid email address(es) in '{field_name}': {formatted}. "
+            f"Each entry must be a valid email address (e.g. user@example.com)."
+        )
+
+
 def _make_mime_text(
     body: str,
     content_type: Optional[Literal["auto", "plain", "html"]] = None,
@@ -100,6 +122,13 @@ async def create_mime_message(
 ) -> str:
     """Create a MIME message with attachments and return base64-encoded raw message."""
 
+    # Validate all recipient lists before building the MIME message
+    validate_email_recipients(input_data.to, "to")
+    if input_data.cc:
+        validate_email_recipients(input_data.cc, "cc")
+    if input_data.bcc:
+        validate_email_recipients(input_data.bcc, "bcc")
+
     message = MIMEMultipart()
     message["to"] = serialize_email_recipients(input_data.to)
     message["subject"] = input_data.subject
@@ -1167,6 +1196,14 @@ async def _build_reply_message(
         references.append(headers["message-id"])
 
     # Create MIME message
+    # Validate all recipient lists before building the MIME message
+    if input_data.to:
+        validate_email_recipients(input_data.to, "to")
+    if input_data.cc:
+        validate_email_recipients(input_data.cc, "cc")
+    if input_data.bcc:
+        validate_email_recipients(input_data.bcc, "bcc")
+
     msg = MIMEMultipart()
     if input_data.to:
         msg["To"] = ", ".join(input_data.to)

autogpt_platform/backend/test/blocks/test_gmail.py

@@ -3,7 +3,7 @@ from unittest.mock import Mock, patch
 
 import pytest
 
-from backend.blocks.google.gmail import GmailReadBlock
+from backend.blocks.google.gmail import GmailReadBlock, validate_email_recipients
 
 
 class TestGmailReadBlock:
@@ -250,3 +250,43 @@ class TestGmailReadBlock:
 
         result = await self.gmail_block._get_email_body(msg, self.mock_service)
         assert result == "This email does not contain a readable body."
+
+
+class TestValidateEmailRecipients:
+    """Test cases for validate_email_recipients."""
+
+    def test_valid_single_email(self):
+        validate_email_recipients(["user@example.com"])
+
+    def test_valid_multiple_emails(self):
+        validate_email_recipients(["a@b.com", "x@y.org", "test@sub.domain.co"])
+
+    def test_invalid_missing_at(self):
+        with pytest.raises(ValueError, match="Invalid email address"):
+            validate_email_recipients(["not-an-email"])
+
+    def test_invalid_missing_domain_dot(self):
+        with pytest.raises(ValueError, match="Invalid email address"):
+            validate_email_recipients(["user@localhost"])
+
+    def test_invalid_empty_string(self):
+        with pytest.raises(ValueError, match="Invalid email address"):
+            validate_email_recipients([""])
+
+    def test_invalid_json_object_string(self):
+        with pytest.raises(ValueError, match="Invalid email address"):
+            validate_email_recipients(['{"email": "user@example.com"}'])
+
+    def test_mixed_valid_and_invalid(self):
+        with pytest.raises(ValueError, match="'bad-addr'"):
+            validate_email_recipients(["good@example.com", "bad-addr"])
+
+    def test_field_name_in_error(self):
+        with pytest.raises(ValueError, match="'cc'"):
+            validate_email_recipients(["nope"], field_name="cc")
+
+    def test_whitespace_trimmed(self):
+        validate_email_recipients(["  user@example.com  "])
+
+    def test_empty_list_passes(self):
+        validate_email_recipients([])