Help! My email was 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 pay-as-you-go and pay only for what you use).

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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