Understanding Gmail's spam handling and answering some FAQs

Not much is known about how Gmail handles spam. So it is no wonder that we receive a regular stream of posts in the Gmail Help Community asking about various aspects of Spam. A few common questions would be:

1. Why is only Gmail flagging messages from me as Spam?
2. My domain was used to send out spam in the past but we have removed that address and changed other details. We are no longer being listed under any blacklists. So, why are emails from our domain still being marked as spam?
3. Please whitelist our domain.
4. Spam filter sending messages to other category tabs instead of Primary.
5. My messages to Gmail are not being delivered.
6. Gmail has suddenly started marking emails from my domain as spam. 

I shall include answers to the questions above after discussing how Gmail handles spam and how to best approach resolving these issues.

This blog is divided into three sections:

1. Understanding Gmail's Spam Handling
2. How to approach resolving your issue
3. Answers to the FAQs

Understanding Gmail's Spam Handling

Gmail uses multiple data points (or signals) to identify messages - such as their own algorithm, information from reputed blacklists, crowd-sourcing and user preference. Based on their assessment, Gmail may decide to include extra info next to the sender’s name in the message, collect the message under Spam or deny delivery of the message altogether. 

So what exactly does the above mean? In Gmail's words, a message might be blocked (or considered Spam) if it contains suspicious-looking or spammy text or if the sending IP(/server/domain) has had a history of sending unsolicited messages and that's a whole list of things to look at and figure out should this be happening with your domain. 

To rule out "suspicious-looking or spammy text" you should look at the content of the mail. There are a number of ways a sender can inadvertently confuse Google's automated spam filter: by sending a suspicious-looking or spammy text, images or common links or by sending emails Cc/Bcc'd to large numbers of recipients ("bulk mail") to send out newsletters, invitations, etc. Since spam is sent to many recipients or any other behavior which may mimic that of a spammer, the spam filter is slightly more likely to confuse bulk mail with spam. Check your emails, both body and signature, for spammy content e.g. lots of exclamation marks; "must read"; attention-getting, large and/or gaudy colored text; text the same color as the background; tiny text; [almost] zero height images; links hidden under text, images or links with a different URL; etc. All or any of these may cause Gmail to mark your emails as spam.

With regards to the signature, ensure it is a simple one as a long signature containing details like phone numbers or email addresses, or signatures with pictures or links to websites or social media pages often causes the message to look like spam. So if you are sending out messages to Gmail recipients with an elaborate signature, and getting your emails flagged as spam, check if simplifying your signature to a short, text-based one or eliminating it brings you better results.

The other cause is that the sending IP(/server/domain) has had a history of sending unsolicited messages either due to the mail server been hacked and used to send out spam or lacking implementation to adequately identify and authenticate emails sent from your domain (SPF, DKIM, and DMARC among others) or if the sending server is running in a shared web hosting environment where other domain users may get a chance to influence the reputation of the sending server. If this is the problem, there's very little one can do about it, other than asking them to request the host to move that account to a different server.

Other activities such as your domain being listed as unsafe with Google Safe Browsing, sending out sample phishing messages or test campaigns from your domain, or impersonating another domain or sender without permission can cause your domain’s reputation might be negatively affected, your domain could be added to internet blocklists, and Gmail to categorize the messages as spam

Additionally, Gmail also puts emphasis on the reputation of the sending server based on how Gmail users may have interacted with messages from that domain, that email address or with messages containing similar text, links, and images. So, user action (or crowdsourcing) can impact both aspects of spam detection - what Gmail considers as suspicious-looking or spammy text, images, and links and which domain is sending them.

At times, using the googlemail variant of your Gmail username as a "Send mail as:" alias may trigger the Spam mechanism. So, please check that possibility as well.

Finally, the Spam team in Google is known to make frequent updates to how spam filtering works in their effort to make it a more efficient system and to counter activities related to spamming, spoofing, and phishing. So, it could also be a recent update causing this issue for you.

How to approach resolving your issue

The first suggestion would be to look at the message from Gmail. In Gmail, there is a banner above that message explaining why the message was flagged as spam. Here are a few examples:

Besides the detail offered in the overhead banner, you can also click on the "?" icon at the bottom-right to read about those reasons in detail. 

The Prevent mail to Gmail users from being blocked or sent to spam article is possibly the only source of official information that lists recommendations on how an external domain sending to Gmail recipients should best set up their credentials. Pay careful attention to all suggestions included there (particularly those relevant to your issue) and implement them.

Among other things, check regularly to make sure your domain isn’t listed as unsafe with Google Safe Browsing. To check your domain status, enter your domain in the Safe Browsing site status page. Also, check any domain that’s linked to yours.

Don’t send sample phishing messages or test campaigns from your domain. Your domain’s reputation might be negatively affected, and your domain could be added to internet blocklists.

Don’t impersonate another domain or sender without permission. This practice is called spoofing, and it can cause Gmail to categorize the messages as spam.

If you happen to send out a fairly large number of emails to Gmail users, I would also suggest you to use this Postmaster tool to help locate the issue: Be a better sender. For information related to it, you can peruse the following help articles:

• Postmaster Tools
• Postmaster Tools FAQs
• Feedback Loop

Use these links to (1) troubleshoot and (2) report this issue to Google.

1. Troubleshooting for senders with email delivery issues
2. Sender Contact Form

It is important to bear in mind that when a domain is used to send out spam, it suffers a reputation hit with Gmail which may take some time to get restored because Gmail usually follows a wait-and-watch policy to confirm that the changes introduced are not temporary. It also becomes extra mindful about ensuring the domain in question is following their guidelines. So, do not expect an immediate change of things.

Answers to the FAQs

• Why is only Gmail flagging messages from me as Spam?

It could be simply because of how Gmail tends to identify incoming messages. Contact the recipient to obtain the reason Gmail assigns for flagging your messages as spam and refer to the details included in the second section of this blog on how to approach resolving it.

• My domain was used to send out spam in the past but we have removed that address and changed other details. We are no longer being listed under any blacklists. So, why are emails from our domain still being marked as spam?

As I have mentioned, do not expect an immediate change of things as Gmail usually follows a wait-and-watch policy to confirm that the changes introduced are not temporary. Review the Bulk Sender Guidelines and use the troubleshooter links to ensure things are in order and keep at it. Users have often mentioned in the community about 2-4 weeks waiting period before noticing a positive change.

• Please whitelist our domain.

Gmail doesn't have a whitelisting policy. Whitelisting in Gmail parlance ensures a guaranteed delivery to a user's inbox, which Gmail is unable to promise prior to their assessment of the received message. All incoming messages must go through Gmail's assessment of them before being delivered as determined by the system and any user preference indicated through filters and Gmail settings.

• Spam filter sending messages to other category tabs instead of Primary.

This is not an issue related to the spam filter. The various inbox categories - Primary, Social, Forums, Promotions, and Updates collectively make up the inbox. So, in essence, the emails are being delivered to that inbox, just not under the right category.

To correct, the recipient can just drag and drop one such message from the wrong category to the Primary and click Yes on the confirmation black box for all future messages. 

Otherwise, the surest and quickest way to customize it to your liking would be by creating a filter. Sign in to your account from a desktop browser and open one such email from that sender, click on the 3-dots (more) menu at the right side, click on "Filter messages like this", click on Create filter, Categorize as <Primary>.

You can also apply the filter to other matching messages. This overrides the default Gmail classification to place messages from that sender to where you specify it to be.

• My messages to Gmail are not being delivered.

You would need to look at the bounce-back message for the exact error message included therein. Subsequently, review the Bulk Sender Guidelines and use the troubleshooter links to ensure things are in order and keep at it.

• Gmail has suddenly started marking emails from my domain as spam.

It could be because of how Gmail tends to identify incoming messages or due to any recent changes implemented by the team. Contact the recipient to obtain the reason Gmail assigns for flagging your messages as spam and refer to the details included in the second section of this blog on how to approach resolving it.