SMTPServerDisconnected: Connection unexpectedly closed Email / SMTP Origin: Network Last updated 4 Nov 2026

Python: smtplib.SMTPServerDisconnected: Connection unexpectedly closed

The server closed the connection before your script finished the SMTP conversation — a timeout, blocked IP, TLS mismatch or rate limit.

Exact message you may see smtplib.SMTPServerDisconnected: Connection unexpectedly closedSMTPServerDisconnected Connection unexpectedly closedSMTPServerDisconnected: please run connect() firstConnection unexpectedly closed: timed out
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Seen on: SMTP Python

Meaning

The error is raised at whichever call was in progress (connect, ehlo, starttls, login, sendmail). Run with set_debuglevel(1) to see the last server line. Servers hang up on wrong TLS mode, idle timeouts, too many messages per connection, abuse filters or blocked IPs.

Common causes

  • Wrong TLS mode for the port (SMTP vs SMTP_SSL)
  • Idle timeout on a long-lived connection
  • Server rate limits or blocks the IP
  • Message too large
  • Firewall/antivirus closing the connection

⚡ Quick fix

  1. Enable server.set_debuglevel(1) and read the final reply
  2. Match SMTP/SMTP_SSL with the port
  3. Reconnect for each batch and add timeouts

Detailed fix by platform

Python

  1. with smtplib.SMTP(host, 587, timeout=30) as s: s.set_debuglevel(1); s.ehlo(); s.starttls(); s.ehlo(); s.login(u, p); s.send_message(msg)

How to diagnose

  1. Step — Which call raised?
  2. Debug — Last server line?
  3. Port — Mode matches?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

421 4.7.0 Try again later (rate limited) — SMTP: 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited (Email / SMTP)421 4.7.0 Try again l…450 4.7.1 Greylisted, please try again later — SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html (Email / SMTP)450 4.7.1 Greylisted,…451 4.3.0 Local error in processing — SMTP: 451 4.3.0 Error: queue file write error / 451 Requested action aborted: local error in processing (Email / SMTP)451 4.3.0 Local error…452 4.2.2 Mailbox full / over quota — SMTP: 452 4.2.2 The email account that you tried to reach is over quota (Email / SMTP)452 4.2.2 Mailbox ful…ConnectionRefusedError [Errno 111] (SMTP) — Python: ConnectionRefusedError: [Errno 111] Connection refused — smtplib connect to localhost:25 (Email / SMTP)ConnectionRefusedErro…Helo command rejected: need fully-qualified hostname — SMTP: 504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname (Email / SMTP)Helo command rejected…SMTPAuthenticationError (Python) — Python: smtplib.SMTPAuthenticationError: (535, b'5.7.8 Username and Password not accepted') (Email / SMTP)SMTPAuthenticationErr…SSL: WRONG_VERSION_NUMBER (SMTP port) — SMTP: [SSL: WRONG_VERSION_NUMBER] wrong version number — SMTP over SSL on a STARTTLS port (Email / SMTP)SSL: WRONG_VERSION_NU…SMTPServe…

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