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IMPORTANT UPDATE:

Starting February 1, 2024, senders who send more than 5,000 messages per day to Gmail accounts must meet the requirements in this section.

• Set up SPF and DKIM email authentication for your domain.
• Ensure that sending domains or IPs have valid forward and reverse DNS records, also referred to as PTR records.
• Keep spam rates reported in Postmaster Tools below 0.3%.
• Format messages according to the Internet Message Format standard (RFC 5322).
• Don’t impersonate Gmail From: headers. Gmail will begin using a DMARC quarantine enforcement policy, and impersonating Gmail From: headers might impact your email delivery.
• If you regularly forward email, including using mailing lists or inbound gateways, add ARC headers to outgoing email. ARC headers indicate the message was forwarded and identify you as the forwarder. Mailing list senders should also add a List-id: header, which specifies the mailing list, to outgoing messages.
• Set up DMARC email authentication for your sending domain. Your DMARC enforcement policy can be set to none.
• For direct mail, the domain in the sender's From: header must be aligned with either the SPF domain or the DKIM domain. This is required to pass DMARC alignment.
• For subscribed messages, enable one-click unsubscribe and include a clearly visible unsubscribe link in the message body.

If you send more than 5,000 emails per day before February 1, 2024, follow the guidelines in this article as soon as possible. Meeting the sender's requirements before the deadline may improve your email delivery. If you don’t meet the requirements described in this article, your email might not be delivered as expected or might be marked as spam.

Market share and history of MBP

Gmail.com, the email service provided by Google, was first launched on April 1, 2004. Initially, it was only accessible through invitation, but in 2007, it became available to the general public. Since then, Gmail has gained widespread popularity and has become one of the leading email services worldwide. It offers various features such as ample storage space, efficient spam filtering, and a user-friendly interface, contributing to its success and providing users with a convenient and enjoyable email experience.

Anti-Spam Measures

Google's approach to blocklists used in their messaging infrastructure is not disclosed publicly. In order to safeguard its messaging system, Gmail depends on its own internal filter. While Gmail does not overtly rely on public blocklists to filter or reject emails, there is a possibility that it incorporates data from reliable blacklists, such as Spamhaus, in certain ways.

Feedback Loop

Unlike traditional Feedback Loops, which report on individual email recipients marking an email as spam, Gmail's Aggregate FBL reports on patterns of user behavior across a large number of recipients and are accessible via Google Postmaster Tools.

We recommend implementing The List-Unsubscribe header should include both “https://” and “mailto://”. The “https://” link should not require further validation or clicks from the recipient for unsubscription. This is a standard header which is read by ISPs systems to unsubscribe recipients from further mailing. The header is not visible to recipients. Recently, the industry has adopted a new header to avoid false-positives of https:// link via List-Unsubscribe by inserting another header called “List-Unsubscribe-Post header”. Google has published implementation guidelines at https://support.google.com/mail/answer/81126?hl=en

Gmail Postmaster Technical & SMTP Tweaks

We recommend Author Domain Signature and domain alignment in both SPF and DKIM. The use of “shared” or “url shortening domains” should be avoided.

Gmail allows the number of connections and messages based on the reputation. If both domain and IP reputation is good, you can open as many connections as needed (per sending IP) to deliver thousands of messages in the shortest time. We have witnessed more than a million messages delivered per day with an IP address.

If you are a new sender with a neutral reputation, we recommend starting with a low volume and slowly raise both the volume and speed while closely monitoring the Google Postmaster tools reputation indicators.

We also recommend configuring MTA to consolidate Google Workspace (formerly G Suite) related domains into a shared queue/ connection for better resource management.

Common Error SMTP Messages

550-5.1.1 The email account that you tried to reach does not exist. Please try 550-5.1.1 double-checking the recipient's email address for typos or 550-5.1.1 unnecessary spaces.

The bounce message refers to a non existing recipient. Such recipients should be treated as a hard bounce, not be retried and removed immediately from the mailing list after the first occurrence.

452 4.2.2 The recipient's inbox is out of storage space. Please direct the recipient to https://support.google.com/mail/?p=OverQuotaTemp

The bounce message refers to a recipient that’s running out of disk space. Such recipients should be marked as soft bounce and suppressed after 3-5 consecutive occurrences over a period of 4-6 weeks.

550 5.7.1 [XXX.XXX.XXX.XXX] Gmail has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. i6-20020adfefc6000000b0032f7ea36ae5si1894742wrp.999 - gsmtp

It commonly happens when a sending domain has no prior mailing history or has a poor reputation.

550 5.7.1 [XXX.XXX.XXX.XXX] Gmail has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. Please visit https://support.google.com/mail/?p=UnsolicitedMessageError for more information. m5-20020a5d6245000000b0032da4610d9fsi6578007wrv.460 - gsmtp

The error message is commonly seen in response to a message that’s previously detected as spam, marked by users as spam. It’s unlikely, the same message will be delivered from a different IP or domain.

421 4.7.28 Gmail has detected an unusual rate of unsolicited mail containing one of your URL domains. To protect our users from spam, mail with the URL has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. b12-20020adfee8c000000b0032f7c14ae6asi7507261wro.896 - gsmtp

421 4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your DKIM domain [XXXXXX]. To protect our users from spam, mail sent from your domain has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. m12-20020a056000174c00b0032f7ee71f60si7625717wrf.296 - gsmtp

421 4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your SPF domain [XXXXXX]. To protect our users from spam, mail sent from your domain has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. b4-20020a05600c150400b003fef979a20csi401526wmg.129 - gsmtp

Since November 22, 2023, these three SMTP error messages are seen in response to a sender that did not respect the new Google Bulk Email Senders Guidelines that will be applied on February 1st, 2024.