Common MTA error messages and resolution


  • Unauthorized mail relaying is not allowed
    • Error Message: 550 #5.1.0 Unauthorized relaying attempt by IP [IP Address] rejected. See http://postmaster.frontiernet.net.
    • Solution: Our incoming mail servers will not act as open relays. Please relay through your authorized mail server.
    • Error Message: 554 <e-mail address>: Relay access denied for <hostname><[IP address]>
    • Solution: You must authenticate your outgoing (SMTP) session.
    • Solution: Metro-E/T-1/Cyber Center customers, please contact your sales team for information on relaying through our servers.

  • Email may not exceed the maximum message size (currently 25MB)
    • Error Message: 552 Message size exceeds file system imposed limit
    • Error Message: 552 Error: message too large
    • Solution: Reduce the size of the e-mail message (including attachments). Alternatively, use a different transfer medium (such as ftp) that is more appropriate for large file transfer.

  • Sender and recipient hostname MX record and NS records cannot be in unallocated or non-routable IP address space. Wild-card addresses in use at some domain registries are not allowed.
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: Unallocated address in MX or NS records (<IP block>)
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: bogus address in MX or NS records (<IP block>)
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: loopback address in MX or NS records (<IP block>)
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: RFC1918 address in MX or NS records (<IP block>)
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: multicast address in MX or NS records (<IP block>)
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: wild-card .nu in MX or NS records (<IP block>)
    • Error Message: 554 <hostname>: (Sender address|Recipient address) rejected: wild-card .com and .net in MX or NS records (<IP block>)
    • Solution: For sender address errors (i.e. you are not a Frontier customer and you are sending to a Frontier customer), See your postmaster or hostmaster for assistance.
    • Solution: For recipient address errors (i.e. you are a Frontier customer sending mail to a non-Frontier customer), check the e-mail address that you are sending to. If you are attempting to deliver mail to our servers, you may have unallocated or reserved IP space in your DNS records. Contact the postmaster of that domain for additional assistance.

  • Connections sent through http proxies will be blocked
    • Error Message: 221 Error: Forbidden command. Goodbye.
    • Solution: Mail should not be sent through http proxies.

  • Connections from customers sending spam will be blocked.
    • Error Message: 553 sorry, your mail (<IP Address>) is rejected. Your IP has been used to send spam or propagate viruses.
    • Solution: Run anti-virus software, and/or disable any unknown software.

  • Mail servers without reverse DNS (i.e. the sending IP address does not resolve to a host name) will be blocked. Mail servers without matching forward and reverse (A and PTR) DNS entries may be blocked or rate limited.
    • Error Message: 554 Client host rejected: cannot find your hostname, [<IP Address>]
    • Error Message: 554 Access from [<IP Address>] has been rejected. Please verify that your IP has a PTR record and/or reference the following URL for more information: http://postmaster.frontiernet.net. If you believe that this failure is in error, please contact intended recipient via alternate means.
    • Solution: Contact your ISP, mail, or DNS administrator.

  • Frontier-maintained IP block list of known spam sending mail servers. If the sender's mail server IP address matches this list, e-mail will be denied.
    • Error message: 553 sorry, your mailserver (<IP Address>) is rejected. Your IP or netblock has sent spam to Frontier.
    • Solution: Contact your ISP or mail administrator. They can then contact Frontier technical support for more information.

  • Messages from <null> senders will be blocked if sent to more than one recipient.
    • Error message: 550 <address>: Recipient address rejected: Multi-recipient bounce
    • Solution: Read RFC2821. Bounce messages with more than one recipient are not allowed.

  • Improperly formatted or unknown envelope sender e-mail addresses will be blocked.
    • Error message: 553 #5.5.4 Domain required for sender address. See http://postmaster.frontiernet.net.
    • Solution: Malformed envelope senders. Check your return e-mail address.
    • Error message: 451 #4.1.8 Domain of sender address <e-mail address> does not resolve. See http://postmaster.frontiernet.net.
    • Solution: Senders domain does not resolve. Check to ensure the proper DNS records have been published for your domain.
    • Error message: 553 #5.1.8 Domain of sender address <e-mail address> does not exist. See http://postmaster.frontiernet.net.
    • Solution: Senders domain does not exist. Check your return e-mail address.

  • Improperly formatted recipient e-mail addresses will be blocked.
    • Error message: 504 <address>: Recipient address rejected: need fully-qualified address
    • Error message: 501 Bad address syntax
    • Solution: Check the recipient (To:) e-mail address.

  • Too many recipients.
    • Error message: 452 Error: too many recipients
    • Solution: Reduce the number of recipients in a single e-mail message. Consider using software designed for sending messages to a large number of recipients. Review Frontier spam policy and Best Practices information.

  • Sender address rejected: User unknown.
    • Error message: 554 <address>: Sender address rejected: User unknown.
    • Solution: Check your mail client configuration to ensure that your valid frontiernet.net, frontiernet.com, citlink.net, citlink.com, newnorth.net, ctaz.com, or ctaz.net account is in use.

  • Recipient address rejected: User unknown.
    • Error message: 550 #5.1.0 Address rejected <e-mail address>.
    • Solution: This is not a valid frontiernet.net, frontiernet.com, citlink.net, citlink.com, newnorth.net, ctaz.com, or ctaz.net account. Please check the address and try again.

  • Message rate limit exceeded
    • Error Message: 452 <[IP address]> exceeded hourly message threshold
    • Error Message: 452 <[IP address]> exceeded daily message threshold
    • Solution: Check your machine for viruses or trojans; your computer may be sending out spam without your knowledge. Reduce the amount of mail you send per hour or per day. Business class service has higher limits than Residential; if you need to send more mail as a business requirement, consider upgrading.

  • Last, Frontier customers may choose to utilize Frontier Anti-Spam to further block messages that they do not wish to receive. The level of blocking is customized by each user after many different tests are performed on received e-mail.
    • Error message: 550 5.7.1 <recipient's e-mail address>: Recipient address rejected: <sender's e-mail address> is on <recipient's e-mail address>'s block list. See http://postmaster.frontiernet.net
    • Solution: Contact the recipient by other means and ask them to add you to remove you from their block list.

DNSBL error messages and suggested resolutions

    Frontier uses a varied group of publicly available block lists to refuse mail from known spammers, compromised (insecure) computers and servers, dynamically assigned IP addresses (i.e. dialup and ADSL), and certain domains that do not have appropriate DNS entries or contact information. These block lists are used to protect our customers from spam and viruses. NOTE: Frontier utilizes these lists, but does not maintain them. Read the instructions listed at the blocking list's information page. You may need to contact your ISP and have them assist you. Do forward the entire error message to your ISP or mail administrator - it is critical for troubleshooting.

    Frontier block lists
    • Error message: 554 sorry, <hostname>[<IP address>] - IP has been used to send spam or propagate viruses. See http://postmaster.frontiernet.net/error.html#frontiernet
    • Error message: 554 sorry, <hostname>[<IP address>] - IP or netblock has sent spam to Frontier. See http://postmaster.frontiernet.net/error.html#frontiernet
    • Error message: 554 sorry, <hostname>[<IP address>] - Block listed in SPEWS #S2680. See http://postmaster.frontiernet.net/error.html#frontiernet
    • Description: This error message means that the mail server sending mail to the Frontier mail servers was, and may still be, sending spam. Spam may be sent through your computer/server without your knowledge if it has been compromised by a virus/trojan.
    • Solution: Stop sending spam or fix your system so that it is no longer compromised. Contact your ISP, mail administrator, or system administrator for more assistance.

    Senderbase Reputation Data
    • Error message: 554 Access from [<IP Address>] has been rejected. Please verify that your IP has a PTR record and/or reference the following URL for more information: http://postmaster.frontiernet.net. If you believe that this failure is in error, please contact intended recipient via alternate means.
    • Description: Real-world data such as spam traps, customer complaints, and spam filtering software is suggesting that the host IP is or is likely to deliver unwanted mail to our customers. We have opted to block and/or rate limit mail coming from this IP.
    • Solution: Contact your ISP, mail administrator, or system administrator for more assistance and/or see http://www.senderbase.org/search?searchString=[<IP Address>]

For more information on spam, and how you can do your part in reducing it, visit these sites: