Accuweb Hosting sends all outgoing emails through MailChannels, a top service that ensures your emails are delivered safely and reach the recipient.
Sometimes, your email might not get through, and you may receive an error message with the subject: 'Mail delivery failed: returning message to the sender.' Wondering why this happens and how to fix it? This article will explain 14 reasons emails fail to be delivered and how to solve these problems.
When you get a return email, the sending mail server (SMTP server) couldn't send your email correctly, or the recipient Mail server could not accept the email. The email will tell you:
- The email address that failed to receive the message.
- The specific reason for the failure.
Emails fail for many reasons, but the most common is that the recipient's email address is invalid. This could mean the address no longer exists, was never valid, or has been blocked. Also, your email account or domain might be blocked due to a lot of spam originating from your account.
Example of bounce-back email
Mail Control Data:
mailnull 47 12
<>
1726765366 0
-received_time_usec .479961
-received_time_complete 1726765366.482381
-ident mailnull
-received_protocol local
-body_linecount 1108
-max_received_linelength 262
-allow_unqualified_recipient
-allow_unqualified_sender
-frozen 1726765367
-localerror
-tls_resumption A
XX
1
info@accuwebtraining.com
Date:
Thu, 19 Sep 2024 11:02:46 -0600
From:
Mail Delivery System
To:
info@accuwebtraining.com
Subject:
Mail delivery failed: returning message to sender
Auto-Submitted:
auto-replied
Content-Type:
multipart/report; report-type=delivery-status; boundary=1726765366-eximdsn-1517585319
Message-Id:
MIME-Version:
1.0
Received:
from mailnull by ssdlinux36.accuwebhosting.com with local (Exim 4.96.2)
id 1srKYI-007x7u-1X
for info@accuwebtraining.com;
Thu, 19 Sep 2024 11:02:46 -0600
References:
<4uo56h671kmm.0E19-qGTrsUN5qlGKBxQHg2@tracking.accuwebtraining.com>
X-Failed-Recipients:
nick.accuwebhosting@hotmail.com
--1726765366-eximdsn-1517585319
Content-type: text/plain; charset=us-ascii
This message was created automatically by mail delivery software.
A message you sent could not be delivered to one or more
recipients. This is a permanent error. The following address(es) failed:
nick.accuwebhosting@hotmail.com
(ultimately generated from nick@demovpstest.com)
host smtp.mailchannels.net [35.164.224.12]
SMTP error from remote mail server after end of data:
550 5.7.1 [CS] Message blocked. If this is a false positive, please report this to your hosting service provider. See https://console.mailchannels.net/insights/bounce?auid=accuwebhosting&sender=info%40accuwebtraining.com&txid=508cc3432a9f6c7b
--1726765366-eximdsn-1517585319
Content-type: message/delivery-status
Reporting-MTA: dns; ssdlinux36.accuwebhosting.com
Action: failed
Final-Recipient: rfc822;nick@demovpstest.com
Status: 5.0.0
Remote-MTA: dns; smtp.mailchannels.net
Diagnostic-Code: smtp; 550 5.7.1 [CS] Message blocked. If this is a false positive, please report this to your hosting service provider. See https://console.mailchannels.net/insights/bounce?auid=accuwebhosting&sender=info%40accuwebtraining.com&txid=508cc3432a9f6c7b
--1726765366-eximdsn-1517585319
Content-type: message/rfc822
Return-path:
Received: from np182.mxout.mta3.net ([216.169.98.182]:33225)
by ssdlinux36.accuwebhosting.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
(Exim 4.96.2)
(envelope-from )
id 1srKY8-007wfS-2d
for nick@demovpstest.com;
Thu, 19 Sep 2024 11:02:45 -0600
DKIM-Signature: v=1; a=rsa-sha256; d=accuwebtraining.com; s=api;
c=relaxed/simple; t=1726765265;
h=from:date:subject:reply-to:to:list-unsubscribe:list-unsubscribe-post:
mime-version;
bh=X83f42ZnYjgRGHUzyfPv7AiU/CWMoPp2IZT+8/ZzmLk=;
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eHfl5YTKngth6oFgJ+Q=
DKIM-Signature: v=1; a=rsa-sha256; d=elasticemail.com; s=api;
c=relaxed/simple; t=1726765265;
h=from:date:subject:reply-to:to:list-unsubscribe:list-unsubscribe-post;
bh=X83f42ZnYjgRGHUzyfPv7AiU/CWMoPp2IZT+8/ZzmLk=;
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THTuViBcdj2hr+auLNk=
From: ACTS Missions
Date: Thu, 19 Sep 2024 17:01:05 +0000
Subject: We want to hear from YOU!
Message-Id: <4uo56h671kmm.0E19-qGTrsUN5qlGKBxQHg2@tracking.accuwebtraining.com>
Reply-To: ACTS Missions
Sender: ACTS Missions
To: nick@demovpstest.com
List-Unsubscribe:
=?us-ascii?q?=3Cmailto=3Aunsubscribe+0E19-qGTrsUN5qlGKBxQHg2=40bounces=2Eelasticem?=
=?us-ascii?q?ail=2Enet=3Fsubject=3Dunsubscribe=3E=2C?=
=?us-ascii?q?_=3Chttp=3A=2F=2Ftracking=2Eactsmissions=2Eorg=2Ftracking=2Funsubscribe=3Fmsgid=3D?=
=?us-ascii?q?0E19-qGTrsUN5qlGKBxQHg2&c=3D1797735252475740514=3E?=
List-Unsubscribe-Post: List-Unsubscribe=One-Click
X-Msg-EID: 0E19-qGTrsUN5qlGKBxQHg2
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="=-eZCfVEeEp2jdAPOMefwBPDjN9S9Azc8Ny3WKzQ=="
X-Spam-Status: No, score=0.3
X-Spam-Score: 3
X-Spam-Bar: /
X-Ham-Report: Spam detection software, running on the system "ssdlinux36.accuwebhosting.com",
has NOT identified this incoming email as spam. The original
message has been attached to this so you can view it or label
similar future email. If you have any questions, see
root\@localhost for details.
Content preview: September 19, 2024
[...]
Content analysis details: (0.3 points, 5.0 required)
pts rule name description
---- ---------------------- --------------------------------------------------
0.0 URIBL_BLOCKED ADMINISTRATOR NOTICE: The query to URIBL was
blocked. See
http://wiki.apache.org/spamassassin/DnsBlocklists#dnsbl-block
for more information.
[URIs: fonts.googleapis.com]
0.0 RCVD_IN_VALIDITY_CERTIFIED_BLOCKED RBL: ADMINISTRATOR NOTICE:
The query to Validity was blocked. See
https://knowledge.validity.com/hc/en-us/articles/20961730681243
for more information.
[216.169.98.182 listed in sa-accredit.habeas.com]
0.0 RCVD_IN_VALIDITY_RPBL_BLOCKED RBL: ADMINISTRATOR NOTICE: The
query to Validity was blocked. See
https://knowledge.validity.com/hc/en-us/articles/20961730681243
for more information.
[216.169.98.182 listed in bl.score.senderscore.com]
-0.0 SPF_PASS SPF: sender matches SPF record
0.0 HTML_MESSAGE BODY: HTML included in the message
0.0 HTML_FONT_LOW_CONTRAST BODY: HTML font color similar or
identical to background
-0.1 DKIM_VALID_AU Message has a valid DKIM or DK signature from
author's domain
0.1 DKIM_SIGNED Message has a DKIM or DK signature, not necessarily
valid
-0.1 DKIM_VALID Message has at least one valid DKIM or DK signature
-0.1 DKIM_VALID_EF Message has a valid DKIM or DK signature from
envelope-from domain
0.5 URI_IMG_CWINDOWSNET Non-MSFT image hosted by Microsoft Azure
infra, possible phishing
X-Spam-Flag: NO
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--=-eZCfVEeEp2jdAPOMefwBPDjN9S9Azc8Ny3WKzQ==--
--1726765366-eximdsn-1517585319--
How to Use This Information:
The return email can list many reasons, and some errors may happen more often. Use the keywords from the error message to search this article and find the solution (sometimes, no action is needed at all).
The specific reason for the return email:
1. Recipient address doesn't exist
Cause: The recipient's email account doesn't exist, or their server can't find the email address.
Possible error keywords: No such user, user unknown, unknown or illegal alias, account inactive, user not found, invalid address, invalid recipient, not a valid mailbox, mailbox cannot be delivered.
Solution: The email address might be incorrect. Double-check the address or ask the recipient for the correct one. After confirming, resend the email.
2. Insufficient space in the recipient's mailbox
Cause: The recipient's mailbox doesn't have enough space to receive new emails.
Possible error keywords: Mailbox is full, exceeded, hard limit.
Solution: The recipient's mailbox is full. Contact them using another method (like a phone call or Google chat) and ask them to delete old or unnecessary emails to make space for new ones.
3. Recipient's mailbox is disabled
Cause: The recipient's mailbox is disabled or unavailable, possibly because it hasn’t been used for a long time or the account was frozen (e.g., for non-payment).
Possible error keyword: Disabled.
Solution: Ask the recipient to contact their system administrator to resolve the issue with their mailbox.
4. Error in domain name resolution
Cause: There’s an issue with the recipient’s server or domain name, and the internet's DNS system can’t properly find the recipient’s mail server IP, causing a delivery error. It could also be a network problem on your server or a DNS failure.
Possible error keywords: Host not found, bad host, domain unreachable, connection refused.
Solution: First, check if the recipient’s email address is spelled correctly. If there’s an error, fix it and resend the email. This is a common issue with returned emails.
5. Mail is filtered
Cause: The recipient's server has restrictions, and your email doesn't meet their rules or requirements.
Possible error keywords: Not welcome, content rejected, message blocked, recipient rejects, no welcome message from ****, mail refused/box is busy.
Solution: The recipient has set up filters that block emails based on certain details, such as the sender's name, email address, subject, content, or attachments. This causes your email to be rejected.
Try adjusting the content of your email, like changing the subject. If the problem persists, contact the recipient another way and share the error message with them. They can help identify which filter caused the issue so you can make the necessary changes and resend the email.
6. Email size exceeds the maximum limit
Cause: The recipient’s email system rejected the message because it was too large.
Possible error keywords: Exceeds, maximum message size, data size.
Solution: This error happens when the recipient’s mail system doesn’t allow too large emails. In that case, contact the recipient and ask them to increase the email size limit or share the attachments in another mode, like transferring the file through an online transfer site (wetransfer.com).
7. Unrecognized command
Cause: The recipient’s server doesn’t recognize a command from the sender’s mail system, causing an error.
Possible error keywords: Error in command, syntax error, error in email address format.
Solution: This usually happens when the sender’s software or server sends a command that the recipient’s system can’t understand. It can also occur if there are illegal characters like double quotes ("), slashes (/), or carets (^) in the recipient’s email address.
Ensure the email address is correctly formatted and includes no illegal characters, then try resending the email.
8. Failed to connect to the recipient's server
Cause: The system found the recipient’s mail server, but it couldn't get a response due to internet issues or problems with the recipient’s server (such as the MTA service). This means the recipient's server isn’t responding.
Possible error keywords: Connection timed out, connect to xxx.xxx.xx.xx: 25 failure, error sending mail body to ******, failure welcome message from.
Solution: Use the command "nslookup" to find the recipient’s mail server IP from the MX record and "telnet" to check if there’s a network issue or if port 25 on the recipient's server isn’t responding. Usually, the recipient's system administrator will fix the problem quickly, so you can try resending the email later. However, in some cases, especially with certain countries' email providers, the issue may last longer. If it fails for over a day, try contacting the recipient by another method.
Ensure your system can connect to external hosts correctly and test if you can connect to the recipient’s server (look for a response starting with "220").
9. Email is rejected
Cause: The recipient’s mail service provider rejected the email.
Possible error keywords: banned, not in my list of allowed rcpthosts, domain not welcome, connection refused, relaying denied, spam, spammers.
Solution: The email was rejected because the sender’s address, domain, or IP is on the recipient’s blocklist.
10. Anti-spam blacklist
Cause: The sending server’s address is on a blacklist from anti-spam organizations, so the email is rejected.
Possible error keywords: Mail from ... refused, see http://**************.
Solution: Some anti-spam organizations block IP addresses from specific regions or countries, even if it’s unjustified. The return email usually includes a link. You can follow the link to apply for your mail server’s IP address to be removed from the blacklist.
11. DNS reverse lookup
Cause: To prevent spam, some mail servers do a DNS reverse lookup on the sender's email address. If the sender’s DNS settings are correct, the email goes through. If not, the email is blocked.
Possible error keywords: Can't verify FROM domain in DNS, domain does not exist.
Solution: This issue usually happens with business email accounts using their domain names. The domain's SOA and MX records must be correctly set up and accessible. If these records are incomplete or the mail server fails during the reverse lookup, the recipient’s server will block the email. Notify your domain administrator to check and fix the DNS records, or wait for the domain server to be back online before resending.
12. Too many recipients in a single email
Cause: There are too many recipients for one email.
Possible error keywords: Too many, less than xx.
Solution: Some mail systems limit the number of recipients for a single email. If you send the same email to many people at once, the recipient's server may reject it or send an error like "Too many recipients." Try limiting the number of recipients to 20-30 per email. If you have a large list, send the emails in smaller batches.
13. Relay sending error
Cause: Webmail is trying to use relay sending, but an issue with the relay settings is preventing the email from being sent.
Possible error keyword: Relay server said: 535 auth failure.
Solution: Check the relay settings to ensure the authentication account and password are correct. Also, verify the relay server address and check if the server can connect to the relay server properly.
14. IP reverse lookup
Cause: Some mail servers perform a reverse DNS lookup on the sender’s IP address to prevent spam. The email is accepted if the IP is resolved correctly using a PTR record.
Possible error keywords: It doesn't have the correct PTR record, and I cannot find your hostname.
Solution: Ensure your mail server has the correct IP reverse DNS setup with a PTR record.
Conclusion:
Mail delivery failures with the message "Returning Message to Sender" can happen for many reasons, like wrong recipient addresses, full mailboxes, server connection issues, too-large emails, or spam filters. Other causes include incorrect domain or IP settings, relay errors, or the sender’s IP being blocked. By looking at the error message and keywords in the returned email, you can figure out the problem and how to fix it.