When forwarding emails through Postfix to Gmail accounts, many administrators encounter bounce messages containing errors like:

550-5.7.1 [2a01:4f8:d12:11c2::2 16] The sender does not meet basic ipv6 
550-5.7.1 sending guidelines of authentication and rdns resolution

This occurs because Google applies strict verification on both the originating server and forwarding server when processing forwarded messages.

Google evaluates three critical factors when accepting forwarded mail:

• SPF Alignment: The forwarding server must have proper SPF records
• Reverse DNS: The server's IP must have matching PTR records
• SRS Rewriting: Forwarded addresses need proper envelope rewriting

First, create a proper SPF record for your forwarding domain:

v=spf1 ip4:192.0.2.1 ip6:2001:db8::1 -all

Key configuration in Postfix's main.cf:

# Enable SPF checking
smtpd_recipient_restrictions = 
    permit_sasl_authenticated,
    permit_mynetworks,
    reject_unauth_destination,
    check_policy_service unix:private/policy-spf

For IPv4:

host -t ptr 192.0.2.1
1.2.0.192.in-addr.arpa domain name pointer mail.yourdomain.com

For IPv6 (critical for Google's verification):

host -t ptr 2a01:4f8:d12:11c2::2
2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.c.1.1.2.1.d.0.8.f.4.1.0.a.2.ip6.arpa domain name pointer mail.yourdomain.com

Install and configure postsrsd:

# Ubuntu/Debian
sudo apt install postsrsd

# Configuration in /etc/default/postsrsd
SRS_DOMAIN=yourdomain.com
SRS_EXCLUDE_DOMAINS=localhost,localdomain
SRS_SEPARATOR==

Postfix integration:

# In main.cf
sender_canonical_maps = tcp:localhost:10001
sender_canonical_classes = envelope_sender
recipient_canonical_maps = tcp:localhost:10002
recipient_canonical_classes = envelope_recipient

The final working configuration requires these components working together:

1. Proper SPF records for your domain
2. Valid rDNS for both IPv4 and IPv6
3. SRS address rewriting
4. DKIM signing (recommended)

Use these tools to verify setup:

# SPF verification
dig TXT yourdomain.com

# rDNS check
dig -x 192.0.2.1
dig -x 2001:db8::1

# SMTP test
telnet gmail-smtp-in.l.google.com 25

---

When Google rejects emails forwarded through your Postfix server with error messages like "The sender does not meet basic IPv6 sending guidelines," it's typically due to three critical authentication failures:

550-5.7.1 [2a01:4f8:d12:11c2::2 16] The sender does not meet basic ipv6 
550-5.7.1 sending guidelines of authentication and rdns resolution

Google evaluates forwarded emails against your server's credentials rather than the original sender's. This creates an SPF validation mismatch. In your mail headers:

Received-SPF: neutral (google.com: 1.1.1.1 is neither permitted nor denied
by best guess record for domain of sender@example.com)

Install pfix-srs on Ubuntu (requires manual installation for older versions):

sudo apt-get install postsrsd
sudo postconf -e "sender_canonical_maps = tcp:127.0.0.1:10001"
sudo postconf -e "sender_canonical_classes = envelope_sender"
sudo postconf -e "recipient_canonical_maps = tcp:127.0.0.1:10002"
sudo postconf -e "recipient_canonical_classes = envelope_recipient"
sudo service postfix restart

Create both SPF and reverse DNS (PTR) records for your mail server:

; SPF Record
yourdomain.com. IN TXT "v=spf1 ip4:1.1.1.1 ip6:abcd:abc:123:4567::8 ~all"

; PTR Record (set at your hosting provider)
2.2.1.1.2.c.1.2.1.d.8.f.4.0.1.0.a.2.ip6.arpa. IN PTR mail.yourdomain.com.

Use these commands to test your setup:

# Check SPF
dig +short txt yourdomain.com

# Check RDNS
dig -x 1.1.1.1
dig -x abcd:abc:123:4567::8

# Test mail delivery
swaks --to user@gmail.com --from external@example.org --server your.mail.server

If problems continue:

1. Verify all Postfix configuration files (/etc/postfix/main.cf, master.cf)
2. Check firewall settings for both IPv4 and IPv6
3. Monitor mail logs in real-time: tail -f /var/log/mail.log