When troubleshooting SMTP transactions with Postfix, I encountered an interesting pattern where an SMTP client (sSMTP) would issue RSET after successful RCPT TO instead of proceeding to DATA. The sequence looked like this:
C: MAIL FROM:<sender@example.com> S: 250 OK C: RCPT TO:<recipient@domain.com> S: 250 OK (queued as ABC123) C: RSET S: 250 OK
After examining RFC 2821 and various SMTP implementations, I found these common scenarios where clients might intentionally use RSET:
- Recipient Verification: Some clients perform recipient verification before sending actual content
- Connection Pooling: Clients maintaining persistent connections might reset state between transactions
- Error Recovery: The client might be implementing a retry mechanism after detecting potential issues
To get more visibility into this behavior, I modified Postfix's logging configuration in master.cf:
smtp inet n - n - - smtpd -o smtpd_verbose=yes -o smtpd_debug=yes
While sSMTP's source didn't explicitly show RSET usage, many SMTP clients implement similar patterns. Here's a Python example demonstrating why a client might use RSET:
def verify_recipient(smtp_server, recipient):
try:
smtp_server.docmd('MAIL FROM:<verify@example.com>')
code, msg = smtp_server.docmd('RCPT TO:<%s>' % recipient)
smtp_server.docmd('RSET') # Clean up verification attempt
return code == 250
except smtplib.SMTPException:
return False
When analyzing these transactions, consider these network factors:
- Packet captures (tcpdump/wireshark) can show exact timing between commands
- Firewall/NAT devices might interfere with SMTP sessions
- TLS negotiation can affect command sequencing
For systematic debugging, I implemented this monitoring script that logs RSET occurrences:
#!/bin/bash
tail -F /var/log/mail.log | \
grep --line-buffered -E 'client=|RSET' | \
awk '/client=/ {client=$0} /RSET/ {print client,$0}'
In SMTP protocol analysis, the RSET command serves as a transaction abort mechanism that clears all stored state (sender, recipients, and buffered data) while maintaining the TCP connection. Unlike QUIT which terminates the session, RSET allows the client to restart the mail transaction from scratch.
The debug logs show this sequence:
220 myserver.example.com ESMTP Postfix EHLO client.example.com 250-myserver.example.com 250-PIPELINING 250-SIZE 10240000 250-VRFY 250-ETRN 250-STARTTLS 250-ENHANCEDSTATUSCODES 250-8BITMIME 250 DSN MAIL FROM:<sender@example.com> 250 2.1.0 Ok RCPT TO:<recipient@example.com> 250 2.1.5 Ok RSET 250 2.0.0 Ok
Through analysis of various SMTP clients and RFC implementations, we've identified these scenarios:
1. Recipient Verification Rollback
Some clients use RSET after testing recipient validity through RCPT TO, especially when implementing fallback mechanisms. Example test case:
# Python smtplib example demonstrating verification pattern
import smtplib
server = smtplib.SMTP('localhost')
server.ehlo()
server.mail('sender@example.com')
try:
server.rcpt('recipient@example.com') # Test recipient
server.rset() # Clear test transaction
# Now start actual transaction
server.mail('sender@example.com')
server.rcpt('recipient@example.com')
server.data()
server.sendmail(...)
finally:
server.quit()
2. SMTP Pipeline Optimization
Modern clients may use RSET when implementing pipelining optimizations gone wrong. The Postfix logs showing dual RSET commands suggest a retry mechanism:
# Typical pipeline flow vs observed behavior Normal: EHLO → MAIL → RCPT → DATA Observed: EHLO → MAIL → RCPT → RSET → (delay) → MAIL → RCPT → DATA
When encountering unexpected RSET commands:
Packet Capture Analysis
Use tcpdump to capture the full conversation:
sudo tcpdump -i lo -w smtp.pcap port 25
Postfix Debug Configuration
Enable detailed protocol logging in master.cf:
smtp inet n - y - - smtpd -o smtpd_verbose=yes -o smtpd_debug=yes
The behavior suggests the sSMTP client might be implementing:
- A premature optimization attempt
- An error recovery pattern
- Compatibility code for broken servers
For closed-source clients, consider implementing an SMTP proxy to log and potentially modify the command flow:
# Basic Python SMTP proxy skeleton
import socketserver
import smtpd
class CustomProxy(smtpd.SMTPServer):
def process_message(self, peer, mailfrom, rcpttos, data):
print(f"From: {mailfrom}\nTo: {rcpttos}\n\n{data}")
# Forward to actual Postfix
with smtplib.SMTP('localhost') as s:
s.sendmail(mailfrom, rcpttos, data)
server = CustomProxy(('0.0.0.0', 2525), None)
server.serve_forever()