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Why is my email blocked?


Email receivers use sophisticated anti-spam mechanisms that sometimes ensnare legitimate email traffic. If you’ve come to this page, you may be the victim of a “false positive”. However, it’s also important to look at your system’s security to make sure your computer and/or network have not been compromised by a spammer.

How spammers affect your IP reputation

Spammers steal user accounts and compromised mail servers using security exploits. Once an account or server has been compromised, large amounts of spam can be sent – all without your knowledge. When email receivers see spam coming from your account or server, they may block you temporarily.

What kinds of error messages will I see if I am blocked?

Anti-spam error messages generally start with a number between 500 and 600, followed by a textual description of the problem. Here are some examples:

Rank Code
1 451 Not currently accepting mail from your ip – psmtp
2 421 STRING (STRING) Unfortunately, some messages from weren't sent. Please try again. We have limits for how many messages can be sent per hour and per day. You can also refer to
3 421 Service unavailable – try again later
4 554 (RTR:BL)
5 475 access from your network is restricted, try later
6 550 Rejected:, listed at for remediation.
7 421 STRING cox blocked – Error Code: CDRBL – Refer to Error Codes section at for more information.
8 571 Email from is currently blocked by Verizon Online's STRING system. The email sender or Email Service Provider may visit and request removal of the block. NUMBER
9 554 5.7.1 – ERROR: Mail refused – <> – See
10 421 4.4.2 service timed out.

How to fix the problem

If your email is being rejected, it’s important to take the following steps before complaining to your service provider:

  1. Update your anti-virus software and conduct a full scan of your system to ensure that a spammer has not installed spam-sending malware on your computer.
  2. Examine the logs on your mail server – look for suspicious activity that may be coming from a compromised user account. Disable any suspicious user accounts that are sending unusual volumes of email.
  3. If you are sending to a mailing list, make sure all the recipient addresses are valid. Invalid addresses are a sign of spamming activity and a common cause for rejection.
  4. Check your domain reputation using one of the following reputation tools:

    If your email messages contain a URL or domain that is blocked by one of these services, you will have difficulty delivering email reliably.

If all else fails

If you’re having trouble delivering hosted email on behalf of your clients, consider using a service like MailChannels. Specialized services may do a better job of delivering your email, by optimizing the way that your email is sent. These services cost very little to use (in many cases, they are free).

If your person-to-person email is being blocked, and you are absolutely sure that a spammer has not compromised your computer or your mail server, then it’s possible that the rejection is a genuine “false positive”. Try sending your message again using slightly different wording. Also, try waiting a while. Most false positives resolve themselves within a few hours.

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  • Avatar
    Barefoot Cell73

    I have tried to e-mail people, but the message was blocked. I changed some of the words, but the message was still blocked.

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  • Avatar
    Sridhar Kakkillaya

    Barefoot Cell73 : Please write to with the following details

    1) Bounceback message or error message you received
    2) Sender email address
    3) Receiver email address
    4) Subject

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  • Avatar
    Barefoot Cell73
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  • Avatar
    genowefa pigwa

    I was trying to send some emails between my two addresses (@consultant and and initially both were identified as spam reciprocally , but after a couple of minutes I received a bounce back notification - my ip address was BLOCKED permanently. To have it unblocked I would have to pay large sums of money to "Microsoft Certified Engineer". Scammers and extortionists have taken over the internet. For shame!

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  • Avatar
    Hayden Holdredge

    I was getting some of my emails blocked then i read an article and tested my email with this website and found my answers. It told me exactly what to do to my DNS records.

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  • Avatar
    Andrew Ford

    My son's teacher sends out a weekly email to the parents of all of her students. For some reason, she has informed me that for the last two weeks her emails have been returned as undeliverable (only my email and my wife's email addresses) as suspected spam. She is sending from a account to mine and my wife's accounts. We live in a small-ish town where the majority of her parents likely have email accounts. What should I do?

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    regina maloka

    How do i unblock my google account

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    Omugave med

    is yoz

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  • Avatar

    I also experience the similar issue when I start to try email marketing at
    I have them to communicate with QQ support team. I was told to have a day limit of 500 emails and 600 messages and increase monthly to 10,0000. It works well for 500,000 QQ emails a months.

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  • Avatar
    vikram sahu

    Good one!
    Initially, when I started sending out my weekly emails, I faced the same issue. Luckily my ESP helped me with email program and kind of email content so that email is not blocked or lands in spam.
    After efforts from both sides, I came to know about my IP is blacklisted and I need to follow few delisted step. I checked 4-5 IP reputation checker, you have already mentioned two of them other are
    1. multivalley rbl ( -- (no delisting steps))
    2. Email blocklist checker ( -- (has delisting steps))
    3. mxtoolbox ( )

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  • Avatar

    Thanks, Vikram
    I also experienced a similar issue. My email was landing in spam due to the IP blocklist. worked for me. It was having delisting steps that helped me to do so and now I can send my email to the target audience.

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  • Avatar
    Roman Regin

    I have also faced similar problem. then i read this blog and getting a solution.

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