diff --git a/tools/gmail.py b/tools/gmail.py
index f299f52..756e791 100644
--- a/tools/gmail.py
+++ b/tools/gmail.py
@@ -5,7 +5,7 @@
 import re
 import smtplib
 from email import message_from_bytes
-from email.header import decode_header as _decode_hdr
+from email.header import decode_header as _decode_hdr, Header as _Header
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
 
@@ -157,6 +157,15 @@
     return False
 
 
+def _enc_header(value: str) -> "_Header | str":
+    """Encode header value as RFC 2047 if it contains non-ASCII characters."""
+    try:
+        value.encode("ascii")
+        return value
+    except UnicodeEncodeError:
+        return _Header(value, "utf-8")
+
+
 def _build_mime(
     to: str, subject: str, body_html: str,
     in_reply_to: str = "", references: str = "",
@@ -166,8 +175,8 @@
     h.body_width = 0
     body_plain = h.handle(body_html)
     msg = MIMEMultipart("alternative")
-    msg["To"] = to
-    msg["Subject"] = subject
+    msg["To"] = _enc_header(to)
+    msg["Subject"] = _enc_header(subject)
     if in_reply_to: msg["In-Reply-To"] = in_reply_to
     if references:  msg["References"] = references
     msg.attach(MIMEText(body_plain, "plain", "utf-8"))