MajorWiki/05-troubleshooting/networking/fail2ban-imap-self-ban-mail-client.md

Mail Client Stops Receiving: Fail2ban IMAP Self-Ban

🛑 Problem

A mail client stops receiving new email on one device while other devices (e.g., phone on cellular) continue to work normally. The mail server is healthy — Postfix is delivering to maildir and Dovecot is running — but the affected device receives no new messages and is never prompted for credentials.

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🔍 Diagnosis

Step 1 — Confirm the mail server is delivering

ssh root@<mailserver> "tail -20 /var/log/maillog | grep 'status=sent'"

If you see status=sent (delivered to maildir), the server is working. The issue is on the client side.

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Step 2 — Check if the client is connecting at all

ssh root@<mailserver> "grep '<client_ip>' /var/log/maillog | tail -10"

If there are zero results, the client is not reaching the server at all — not a credentials or sync issue.

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Step 3 — Get the client's current public IP

Run this on the affected machine:

curl -s https://api.ipify.org

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Step 4 — Check if that IP is banned by Fail2ban

ssh root@<mailserver> "fail2ban-client status postfix-sasl"
ssh root@<mailserver> "fail2ban-client status dovecot-invalid"

Look for the IP in the Banned IP list. If it's there, that's your problem.

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Step 5 — Find when the ban was applied

ssh root@<mailserver> "grep '<client_ip>' /var/log/fail2ban.log | tail -20"

This shows the exact timestamps of auth failures and the ban event.

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✅ Fix

Unban the IP from all relevant jails:

fail2ban-client set postfix-sasl unbanip <IP>
fail2ban-client set dovecot-invalid unbanip <IP>

Mail should resume immediately without restarting any services.

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🔁 Why This Happens

Symptom	Cause
One device stops getting mail, others don't	Each device has a different public IP. Only the banned IP is blocked.
No auth prompt in the mail client	Fail2ban issues a TCP REJECT — the client sees a connection error, not an auth failure, so it doesn't prompt for new credentials. It silently retries on a timer.
Multiple failed attempts before ban	The mail client retried with a previously valid session token or stale auth state, triggering the maxretry threshold.
Ban on postfix-sasl blocks IMAP (port 993)	The postfix-sasl jail monitors ports 25, 465, 587, 143, 993, 110, and 995 — not just SMTP. A failed SMTP auth attempt can ban access to IMAP as well.

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⚠️ Key Notes

• Fail2ban postfix-sasl covers all mail ports — a single jail can lock out SMTP and IMAP simultaneously.
• The affected device's IP is never logged in Dovecot once banned — the rejection happens at the iptables/Fail2ban layer before Dovecot sees the connection.
• Tailscale doesn't help if the mail client connects via the public interface — the ban applies to the public IP, not the Tailscale IP.
• Check when the ban happened: grep '<IP>' /var/log/fail2ban.log shows exact timestamps.

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🔎 Quick Diagnostic Commands

# Get your current public IP (run on affected machine)
curl -s https://api.ipify.org

# Check all Fail2ban jail statuses
fail2ban-client status

# Check a specific jail for a banned IP
fail2ban-client status postfix-sasl
fail2ban-client status dovecot-invalid

# Unban from a specific jail
fail2ban-client set postfix-sasl unbanip <IP>
fail2ban-client set dovecot-invalid unbanip <IP>

# Unban from all jails at once
for jail in $(fail2ban-client status | grep "Jail list" | sed 's/.*://;s/,/ /g'); do
  fail2ban-client set $jail unbanip <IP> 2>/dev/null && echo "Unbanned from $jail"
done

# Find when a specific IP was banned
grep '<IP>' /var/log/fail2ban.log | tail -20