Common Email Bounce Back Messages

Here are a few of the most common bounce back messages that you may encounter during email sending or receiving:

• User unknown in virtual mailbox table
• IP Blacklist
• Reverse DNS Lookup Fail
• SPF Fail
• Mails getting detected as spam by the spam filter
• Header check

User unknown in virtual mailbox table

This is one of the common errors in Postfix mail servers when users send emails to their local mailboxes.

Bounce back: 550 ..: Recipient address rejected: User unknown in virtual mailbox table

Example :
NOQUEUE: reject: RCPT from mail.binetrs.art[62.173.147.70]: 550 5.1.1 <test@example.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<john@titan.email> to=<test@example.com> proto=ESMTP helo=<mail.binetrs.art>

Reason for reject:  This error message shows that emails are not delivered to the mailbox test@example.com. Because the mail server can’t find this email account on the server.

How to fix? Please confirm that there are no typo errors in the email address and is a valid, existing email.

Client IP reputation.

Every email originates from a particular IP address and gets ranked according to their reputation.

Bounce back: Incoming email server rejected the email since the client IP(From IP): xxx.xxx.xx.xx is blacklisted, hence considered as spam

Reason: This is a bounce-back message that you receive stating that the client IP is blacklisted and untrustworthy. 

How to fix? You may have to bring this issue to your ISP's notice and get the IP whitelisted. It's a good practice to regularly determine your IP reputation on a tool like mxtoolbox.com.

Reverse DNS Lookup Fail.

We are doing some additional checks at TITAN Mail server to prevent incoming email spam 

Bounce back: [Default] 450-'4.7.25 Client host rejected: cannot find your hostname, [xxx.xxx.xx.xx]'

Reason: This is a bounce-back message when the emails are getting rejected since the rDNS record of the IP of your email server is not matching with the forward DNS or if there is no rDNS record setup at all. 

How to fix?  If you need a PTR Record(rDNS) set up for your mail server you'll need to contact your mail service provider as this is something they'll set up for you. If your mail server's domain name were mail.yourdomain.com, then you would ask your ISP to set up a reverse PTR record for your mail server and give them the IP address of the server.

SPF Fail.

SPF records are used to prevent any mail forgery, email spoofing, etc.

Bounce back: Message Rejected Due To: Spf Fail

Reason: SPF check is an anti-spam measure to check the SPF (Sender Policy Framework) record of the domain to verify whether the sender is genuine or not.

This SPF record contains the IP addresses of servers that can send emails on behalf of the domain. If the email originates from an IP listed in the SPF record, the recipient server accepts mail.

Likewise, when sending email from an IP address not available in SPF record, it results in a delivery status notification sent to the original sender.

How to fix?  Please ensure your domain's SPF points to the followingTITAN Mail record-
Hostname: "@"
TXT value: "v=spf1 include:spf.titan.email ~all"
TTL: "1800" 

Mails getting detected as spam by the spam filter.

The mail filter will check if the outgoing email is spam, contains a virus or is a phishing email. If it a virus or a spam mail then it is rejected.

Bounce back: [T3758094080] IP xxx.xxx.xx.xx1 spam score 100, rejecting

Reason: This is the error for an incoming email rejection when the mail filter identifies a high spam score for the client IP.

How to fix?  Avoid spam trigger and phishing phrases in your email. Limit your email sending frequency and avoid sending unsolicited bulk emails.

MIME Header Check. 

This is a header check which will essentially check the attachments that have been added in the email. If the file extension format isn't permitted by the incoming email server then the email will be rejected.