Gmail returns 550 5.7.1 "This message is likely unsolicited email" when its spam filters are confident enough that your email is spam to refuse it outright, instead of filing it in the Spam folder. The usual causes are email that Gmail can't tie to your own domain, a poor reputation for your domain or sending server, and too many people reporting your email as spam.
The bounce is permanent, so the email was not delivered. Fix the cause before you send it again.
What the bounce says
Google's list of SMTP errors gives this exact message under 550 5.7.1:
This message is likely unsolicited email. To reduce the amount of spam sent to Gmail, this message has been blocked.
In a real bounce, each line usually starts with "550-5.7.1". "Likely unsolicited" is the part to look for.
Other 550 5.7.1 messages
Gmail uses 550 5.7.1 for several different blocks. Check your wording first.
| The message says | What it means | Where to start |
|---|---|---|
| likely unsolicited email | Gmail's filters think the email is spam. | This article. |
| very low reputation of the sending IP address | The server that sent it has a poor record with Gmail. | Your email service or web host. |
| very low reputation of the sending domain | Your domain has a poor record with Gmail. | Spam complaints and who you send to. |
| This email has been rate limited | Too much email arrived too fast. | Slow down your sending. |
| not RFC 5322 compliant, or a missing From: or Message-ID: header | The email itself is badly formatted. | The app or script that built it. |
Why Gmail decided your email was unsolicited
Google says it blocks some email that looks like spam, because of its wording or the sender's history. These make it far more likely:
- Gmail can't tie the email to your domain. Gmail tracks reputation per domain and per sending server. SPF is a DNS record that lists the servers allowed to send email for your domain. DKIM is a digital signature that proves an email came from your domain and wasn't changed on the way. If neither passes for yourbusiness.com, your good history can't count in your favor.
- Your server shares a poor reputation. Many email services and web hosts send for lots of customers from the same IP addresses, and Google notes that every sender on a shared IP affects the others.
- People report your email as spam. Google asks senders to keep their user-reported spam rate below 0.1% and never let it reach 0.3%.
- You sent to people who didn't ask for it. Bought lists, old lists and pre-ticked sign-up boxes all lead to complaints.
- Your volume jumped. Google warns that suddenly doubling what you send can cause rate limiting or a drop in reputation.
Since November 2025, Google has been tightening enforcement of its sender requirements, so failing email is now more likely to be rejected than filtered to spam.
The 421 4.7.28 version: rate limited
421 4.7.28 is the temporary relative of this bounce. The message says Gmail "has detected an unusual rate" of email, or of unsolicited email, from your IP address, DKIM domain, SPF domain or a domain in your links, and that it has been "temporarily rate limited". Your server retries, so some email may still arrive.
Google's advice: stop sending for at least 10 minutes and find the cause. If the message doesn't say whether your DKIM domain, SPF domain or IP address hit the limit, assume all three did. Then restart with a single connection and add connections one at a time. If you use an email service, pass the bounce to its support team. The steps below apply to both codes.
How to fix it
Step 1: make sure your email is authenticated with your own domain
Send an email from the same app to your private test address. The report shows whether SPF and DKIM pass for yourbusiness.com, whether they match your From address (alignment), and which service sent it.
Google Workspace: in the Google Admin console go to Apps → Google Workspace → Gmail → Authenticate email. Generate a DKIM key for yourbusiness.com, add the TXT record it shows at google._domainkey.yourbusiness.com, then click "Start authentication".
Other services: turn on DKIM signing for yourbusiness.com in the service that sends this email, and add the DKIM record it gives you to your DNS. Most services call this "domain authentication".
If you have no DMARC record, add this TXT record at _dmarc.yourbusiness.com. It starts in monitoring mode (p=none), so nothing changes for your email yet. Change the rua address to a mailbox you read.
v=DMARC1; p=none; rua=mailto:dmarc-reports@yourbusiness.comStep 2: check your spam rate
Set up Google's free Postmaster Tools for yourbusiness.com. Once you send enough email to Gmail, look for your user-reported spam rate and the compliance status dashboard. A spam rate near or above 0.3% is very likely the problem.
Step 3: send only to people who asked, at a steady pace
Remove addresses you bought, scraped or haven't emailed in a long time, and make unsubscribing easy. Google recommends sending at a consistent rate, increasing volume slowly, and cutting back when bounces start.
Step 4: if the server is shared, tell your provider
If the bounce blames the sending IP, or your setup looks clean, send the full bounce to your email service or web host.
Step 5: test, then resend carefully
When SPF and DKIM pass for yourbusiness.com and the Gmail verdict is a pass, resend the bounced email to the people who missed it. Don't resend a whole campaign at once, or you may trigger the same block.
Checked against: Google Workspace Admin Help, Gmail SMTP errors and codes · Gmail Help, Learn how Gmail blocks your messages · Gmail Help, Email sender guidelines · Gmail Help, Email sender guidelines FAQ · Gmail Help, Sender requirements & Postmaster Tools FAQ · Google Workspace Admin Help, Set up DKIM.