• Most popular SMTP responses – updated for August 2012

    August 31st, 2012, by ksimpson

    Pumpkin Spice Muffins at Tim Hortons, Copyright 2010 Ken Simpson.

    A little over two years ago, we posted a list of the world’s most popular SMTP response codes. I’m now pleased to present an update to the list. Using a slightly updated script, we processed about 100GB of message delivery logs at a major North American web hosting provider at which we provide transparent SMTP filtering. We picked up all the response codes, filtered out positive responses like “250 Ok”, and then did a bit of string cleaning to remove serial numbers and other pseudorandom identifiers (you’ll see these as “HEX”, “STRING”, and “NUMBER” in the list below).

    For me, the surprise here is how important AOL still is to email senders, after all these years. If you get error responses like these, chances are you need to improve your outbound spam filtering – we can help.

    Rank Code Frequency (%)
    1 421 4.7.0 [TS01] Messages from 1.2.3.4 temporarily deferred due to user complaints – 4.16.55.1; see http://postmaster.yahoo.com/421-ts01.html 15.02%
    2 421 STRING.r1000.mx.aol.com Service unavailable – try again later 13.78%
    3 421 4.7.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html 8.88%
    4 554 (RTR:BL) http://postmaster.info.aol.com/errors/554rtrbl.html 8.66%
    5 421 4.7.1 [TS03] All messages from 1.2.3.4 will be permanently deferred; Retrying will NOT succeed. See http://postmaster.yahoo.com/421-ts03.html 6.37%
    6 421 4.7.0 [1.2.3.4 10] Our system has detected an unusual rate of 2.86%
    7 554 5.7.1 – ERROR: Mail refused – <1.2.3.4> – See http://postmaster.rr.com/amIBlockedByRR?ip=1.2.3.4 2.81%
    8 421 STRING (STRING) Unfortunately, some messages from 1.2.3.4 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 http://mail.live.com/mail/troubleshooting.aspx#errors. 2.56%
    9 421 4.7.1 : (DNS:NR) http://postmaster.info.aol.com/errors/421dnsnr.html 2.21%
    10 421 4.7.0 [TS01] Messages from 1.2.3.4 temporarily deferred due to user complaints – 4.16.55.1; see http://postmaster.yahoo.com/421-ts01.html 1.83%
    11 450 4.7.1 Connection declined at this time 1.55%
    12 421 4.7.0 [TS02] Messages from 1.2.3.4 temporarily deferred – 4.16.56.1; see http://postmaster.yahoo.com/errors/421-ts02.html 1.48%
    13 421 STRING cox 1.2.3.4 blocked – Error Code: CDRBL – Refer to Error Codes section at http://postmaster.cox.net/confluence/display/postmaster/Error+Codes for more information. 1.46%
    14 421 STRING (STRING) Unfortunately, some messages from 1.2.3.4 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 http://mail.live.com/mail/troubleshooting.aspx#errors. 1.13%
    15 421 4.7.0 [1.2.3.4] Our system has detected an unusual amount of 1.11%
    16 421 Too many concurrent connections from this client. 0.98%
    17 571 Email from 1.2.3.4 is currently blocked by Verizon Online's STRING system. The email sender or Email Service Provider may visit http://www.verizon.net/whitelist and request removal of the block. NUMBER 0.76%
    18 554 STRING cox 1.2.3.4 blocked. Error Code: STRING – Refer to Error Codes section at http://postmaster.cox.net/confluence/display/postmaster/Error+Codes for more information. 0.74%
    19 421 4.2.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html 0.71%
    20 554 STRING.r1000.mx.aol.com ESMTP not accepting connections 0.61%
    21 451 The server has reached its limit for processing requests from your host. 0.60%
    22 550 1.2.3.4 blocked by ldap:ou=rblmx,dc=att,dc=net 0.59%
    23 451 Not currently accepting mail from your ip – psmtp 0.58%
    24 553 5.7.1 [BL21] Connections will not be accepted from 1.2.3.4, because the ip is in Spamhaus's list; see http://postmaster.yahoo.com/550-bl23.html 0.57%
    25 421 too many connections 0.53%
    26 521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html 0.36%
    27 421 4.7.0 [1.2.3.4] Our system has detected an unusual amount of unsolicited 0.34%
    28 521 5.2.1 : (HVU:B2) http://postmaster.info.aol.com/errors/554hvub2.html 0.33%
    29 451 Message temporarily deferred – [70] 0.31%
    30 554 rejected due to spam content 0.30%
    31 421 4.7.0 [1.2.3.4 10] Our system has detected an unusual rate of 0.30%
    32 553 5.7.1 [BL21] Connections will not be accepted from 1.2.3.4, because the ip is in Spamhaus's list; see http://postmaster.yahoo.com/550-bl23.html 0.29%
    33 554 5.2.1 : (HVU:B2) http://postmaster.info.aol.com/errors/554hvub2.html 0.28%
    34 421 mx.fakemx.net Service Unavailable 0.26%
    35 421 4.2.1 : (DNS:NR) http://postmaster.info.aol.com/errors/421dnsnr.html 0.25%
    36 421 4.7.0 [GL01] Message from (1.2.3.4) temporarily deferred – 4.16.50. Please refer to http://postmaster.yahoo.com/errors/postmaster-21.html 0.25%
    37 450 too frequent connects from 1.2.3.4, please try again later. 0.24%
    38 550 STRING (STRING) Unfortunately, messages from 1.2.3.4 weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. 0.23%
    39 421 4.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/421conb1.html 0.23%
    40 421 4.7.1 [TS03] All messages from 1.2.3.4 will be permanently deferred; Retrying will NOT succeed. See http://postmaster.yahoo.com/errors/421-ts03.html 0.22%
    41 421 #4.4.5 Too many connections from your host. 0.20%
    42 501 5.5.4 Syntax: RCPT To:<address> 0.20%
    43 452 try later 0.19%
    44 421 Service not available, closing transmission channel 0.18%
    45 550 Access denied…STRING… 0.18%
    46 421 Service not available 0.17%
    47 550 5.7.1 [1.2.3.4 1] Our system has detected an unusual rate of 0.15%
    48 554 5.7.1 : (RLY:B1) http://postmaster.info.aol.com/errors/554rlyb1.html 0.15%
    49 554 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html 0.15%
    50 451 Message temporarily deferred – [160] 0.15%

    2 Responses to “Most popular SMTP responses – updated for August 2012”

    1. September 05, 2012 at 6:47 am, H. O'Regan said:

      Site aol.com (205.188.103.2) said after data sent: 521 5.2.1 : (CON:B1) http://postmaster.info.aol.com/errors/554conb1.html

      What does this mean – why isn’t my mail reaching my nephew ??? Please advise. He’s 50 tomorrow. (6th Sept. 2012)

      H. O’Regan

      • September 06, 2012 at 4:28 pm, ksimpson said:

        Sorry for the slow reply, H. Chances are, your mail server is being blocked by AOL because of excessive spam originating from your mail server’s IP address. From AOL’s Postmaster page: “The IP address has been blocked due to a spike in unfavorable e-mail statistics.”
         
        Who provides your email service?

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