The log entry status=sent (250 2.6.0 ... Queued mail for delivery) indicates that:

1. The receiving server (the-bank.com.vn) accepted the message
2. The message was placed in their internal queue
3. This is a positive SMTP response (2xx code)

Common reasons for queuing include:

1. Recipient server load balancing
2. Spam filtering processing
3. Internal routing delays
4. Recipient mailbox processing rules

To investigate further, you can:

1. Check Postfix Logs More Deeply

grep "23138874" /var/log/maillog
grep "mrs_lorem@the-bank.com.vn" /var/log/maillog -A 10 -B 5

2. Verify with the Receiving Server

The receiving server's administrators can check their logs using:

grep "InternalId=23138874" /var/log/mail.log

3. Check Message Headers

If the recipient eventually receives the message, examine headers for:

Received: from mail.the-bank.com.vn (internal queuing timestamp)
X-Queue-ID: 23138874

The response breakdown:

Where the message might be:

1. In recipient server's queue (check with postqueue -p)
2. Being processed by spam filters
3. In recipient's Junk folder
4. Delayed by recipient server policies
5. Blocked by recipient-side rules

If the message doesn't arrive within 24 hours:

1. Contact the-bank.com.vn postmaster with InternalId
2. Resend with delivery receipt request
3. Check for bounce messages

Remember that "Queued mail for delivery" means successful acceptance by the receiving server, but final delivery depends on their internal processes.

When analyzing Postfix mail logs, the "Queued mail for delivery" status indicates successful acceptance by the recipient's mail server but doesn't guarantee immediate delivery to the final mailbox. Let's break down the log entries:

Aug  7 17:31:32 mail postfix/smtp[7487]: 020EE4202065: to=, 
relay=mail.the-bank.com.vn[103.11.172.xx]:25, delay=8.3, 
dsn=2.6.0, status=sent (250 2.6.0 ... Queued mail for delivery)

The receiving server (the-bank.com.vn) has:

• Accepted the message into its queue
• Assigned an internal tracking ID (InternalId=23138874)
• Not yet delivered to the recipient's mailbox

To verify complete delivery path:

# Check recipient server's mail queue (if you have access)
postqueue -p

# Grep for the specific message ID
grep "23138874" /var/log/mail.log

# Alternative check for message tracking
swaks --to mrs_lorem@the-bank.com.vn --server mail.the-bank.com.vn --query-last=1h

Common reasons for queued but undelivered mail:

# Content filtering delays
1. Anti-spam scanning
2. Attachment scanning
3. Policy compliance checks

# Example of checking greylisting status (if applicable)
grep "greylist" /var/log/mail.log | grep "23138874"

Adjust these Postfix parameters if facing persistent issues:

# /etc/postfix/main.cf
queue_run_delay = 300s
maximal_queue_lifetime = 5d
bounce_queue_lifetime = 5d

Create a simple monitoring script:

#!/bin/bash
QUEUE_COUNT=$(mailq | grep -v "Mail queue is empty" | wc -l)
if [ $QUEUE_COUNT -gt 100 ]; then
    echo "Warning: High mail queue count detected" | mail -s "Postfix Alert" admin@example.com
fi