SimplerCloud Pte Ltd

If you noticed that your email is being rejected by GMail with below error message on the mail server's logs:

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R=lookuphost T=remote_smtp: SMTP error from remote mail server after end of data: host aspmx.l.google.com [2404:6800:4003:c00::1b]: 550-5.7.1 [2400:c980:0:2:xxxx:xxxx:xxxx:xxxx] Our system has detected that this\n550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records\n550-5.7.1 and authentication. Please review\n550-5.7.1  https://support.google.com/mail/?p=ipv6_authentication_error for more\n550 5.7.1 information. ns1si22138959pbc.169 - gsmtp
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The reason was because the IPv6 address assigned / configured on your servelet doesn't meet their IPv6 sending guidelines regarding PTR record. More information can be found on Google's guidelines at below URL:

https://support.google.com/mail/answer/81126?p=ipv6_authentication_error&rd=1#authentication

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Additional guidelines for IPv6

• The sending IP must have a PTR record (i.e., a reverse DNS of the sending IP) and it should match the IP obtained via the forward DNS resolution of the hostname specified in the PTR record. Otherwise, mail will be marked as spam or possibly rejected.
• The sending domain should pass either SPF check or DKIM check. Otherwise, mail might be marked as spam.

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In summary, basically their requirements are:
1. PTR record will have to be created to map your servelet's IPv6 address to the hostname of your mail server.
2. AAAA record will have to be created to point the hostname of your mail server to the static IPv6 address.
3. If you have SPF record for the domain, you would need to add the IPv6 address into the SPF TXT record.

Here are the details on the steps to resolve the problem:

1. Open a support ticket to request us to create PTR (reverse DNS) record for your servelet's IPv6 address. Please also provide the hostname you would like us to set the PTR record to.

2. Create AAAA record for the hostname pointing to the IPv6 address. If the domain is hosted in SimplerCloud's name servers, you can use the DNS Hosting control panel in our portal to do so. If the domain is hosted elsewhere, you would need to use the DNS provider's DNS Hosting control panel.

3. Modify the SPF record for your domain to add the IPv6 address into the SPF TXT record.

Another alternative, which is not recommended, is to completely disable IPv6 on your servelet's network setings so that you will ensure that you will establish connection using IPv4. However, this is not a recommended option since your servelet might have issues connecting to IPv6-only destinations and servers in the future.