All about Gmail.com: history, best practices and deliverability
To help the sender community, we are delighted to share our extensive knowledge and experiences gained over the years with various Internet Service Providers (ISPs). This particular page focuses on Gmail, one of the most widely used webmail services worldwide. At Postmastery, we have seamlessly integrated with Google Postmaster tools in this case gmail postmaster, utilizing our monitoring tools to provide valuable insights.
IMPORTANT UPDATE:
Last revision JULY 27, 2026
July 23, 2026
Starting November 2025, Gmail is ramping up its enforcement on non-compliant traffic. Messages that fail to meet the email sender requirements will experience disruptions, including temporary and permanent rejections.
Sender requirements
- Domain Authentication: Set up SPF or DKIM authentication for all your sending domains.
- DNS Records: Ensure sending domains or IPs have valid forward and reverse DNS (PTR) records.
- TLS Encryption: TLS connection is required.
- Low Spam Rates: Maintain reported spam rates in Postmaster Tools below 0.3%.
- RFC Compliance: Format all outgoing messages strictly according to the RFC 5322 Internet Message Format standard.
- Header Authenticity: Never impersonate Gmail From: headers, as strict DMARC quarantine policies (currently p=none) will result in delivery failures or rejections.
Bulk Sender requirements
Bulk senders (that send over 5000 emails per day) must comply immediately with the following additional requirements to avoid delivery issues, spam flagging, or message rejection.
- Set up SPF or DKIM authentication for all your sending domains.
- Set up DMARC authentication and enforcement policy can be set to p=none.
- Marketing messages and subscribed messages must support one-click unsubscribe and include a clearly visible unsubscribe link in the message body.
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.
552 5.2.2 The recipient's inbox is out of storage space and inactive. Please direct the recipient to https://support.google.com/mail/?p=OverQuotaPerm ffacd0b85a97d-47q86b04yddsi1524936f8f.253 - gsmtp
The bounce message refers to a recipient that’s running out of disk space and at the same time the inbox has remained untouched and Google has marked it as inactive recipient. Such recipients should be treated as a hard bounce, not be retried and removed immediately from the mailing list after the first occurrence.
550 5.2.1 The email account that you tried to reach is inactive. For more information, go to https://support.google.com/mail/?p=DisabledUser ffadd0b85a97f-47f63wc8079si3801505f8f.373 - gsmtp
The bounce message refers to an inactive recipient. Such recipients should be treated as a hard bounce, not be retried and removed immediately from the mailing list after the first occurrence.
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.
421 4.7.28 Gmail has detected an unusual rate of mail originating from your SPF domain [XXXXXX 35]. To protect our users from spam, mail sent from your domain has been temporarily rate limited. For more information, go to https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. 4fb4d7f45d1cf-69e6fff9a96si10421651a12.219 - gsmtp
Typically, this deferral indicates that the smtp.mailfrom domain (SPF domain) has no prior mailing history. To resolve this, the domain needs to go through a proper warm-up process.
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 4.7.0 [x.x.x.x 21] Gmail has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. For more information, go to https://support.google.com/mail/answer/188131 4fb4d7f45d1cf-69e700089efsi9001439a12.249 - 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
421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message. Gmail requires all bulk email senders to authenticate with SPF. Authentication results: SPF [XXXXXX] with ip: [x.x.x.x] = did not pass To set up SPF for your sending domains, visit https://support.google.com/a?p=setup-spf To learn more about Gmail requirements for bulk senders, visit https://support.google.com/a?p=sender-guidelines. 4fb4d7f45d1cf-69e6fee4c44si10181170a12.35 - gsmtp
Do you have a specific error message you need help with? The Postmastery team is ready to assist, please share some details on our contact form.
Abuse Desk & Tech Support
In the event of any issues pertaining to email deliverability on Gmail's infrastructure, users have the option to contact Gmail via Google sender's escalation form available at Google Postmaster tools.
More information
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