550 5.1.1 Recipient address rejected: User unknown in virtual alias table;
550 5.1.1 Protocol error
This error code ideally suggests that the sender has used SMTP protocol instead of ESMTP, thus the outbound mail of the sender was rejected. In short, the sending client doesn't properly communicate and makes errors in the SMTP protocol.
554 5.7.1 Service unavailable; Client host X.X.X.X blocked using zen.spamhaus.org;
This is the error message that is received of the IP address of the sender is listed at zen.spamhaus.org
For further details, please check http://www.spamhaus.org/zen
OR check http://www.spamhaus.org/query/bl?ip=<your_ipaddress> and contact zen.spamhaus.org to get the IP De-listed.
Our servers do not accept SMTP HELO command as HELO localhost or HELO localhost.localdomain or HELO
550 5.7.1 Service unavailable; client [a.b.c.d] blocked using rbl.mailhostbox.com
rbl.mailhostbox.com is an inhouse RBL zone which we use to protect our services from inbound attacks. If you encounter any of the above messages, you can whitelist the IP using http://whitelist.mailhostbox.com/
To avoid misuse of this feature, an IP is allowed to be white-listed only a defined number of times. If you face any issues feel free to contact our support helpdesk with the details.
There are couple of reasons for this error while sending an email
551 5.7.1 The message was rejected due to classification as Virus,Spam or high bulk ratio. Please see http://support.mailhostbox.com/email-administrators-guide/error-codes
This is a bounce back message that you receive when an email is classified as spam while sending out. If you feel that a genuine email is rejected as spam, you need to send the same email with the full headers and the content to our support team. That email in question will be reviewed manually.
This error comes when your sending mails to remote domain without using smtp authentication.
This is the error message that you get if your domain name exceeds the hourly quota set for the domain name. Here, the term 'hour' refers to the last 60 minutes.
This is the error message that you get if the user (that particular email address) exceeds the hourly quota set. Here, the term 'hour' refers to the last 60 minutes.
This is the error message that is received when you are trying to send an email as a different user. This error message is seen by the users who are using Exchange servers with our system.
You need to add an identity for the same in the webmail. For example,
This error comes if user is suspended in Control Panel.
This error comes if the user is overquota.
This error comes when the sender address is blocked on our server. Generally this is done if spam has originated from this user.
Our inbound servers require that the hostname for the sender server should have valid MX or A records in order to accept emails from it. You need to contact the sending server's email admin to add valid DNS records for his server.
450 4.7.1 Client host rejected: cannot find your reverse hostname
Our inbound servers require reverse hostname for the sender server to be present in order to accept the email. You need to contact the reverse hostname's email admin to add the rDNS to into his server.
This error comes from our incoming mail server if user is receiving excessive amounts of mails.
This occurs when the recipient address is invalid. i.e., not in form of user@domainname OR if the Recipient address is blocked on server.
451 4.3.5 Server configuration problem - try again later;
451 4.7.1 Service unavailable - try again later;
This error most likely occurs if there is a configration error with our servers. If you encounter any of the above messages, please contact our support helpdesk with the details.
452 4.5.3 Error: too many recipients
The error message is encountered when the user is exceeding the limit of more than 50 recipients in an email. The list of recipients is inclusive of To,Cc and Bcc. If you encounter the above error, reduce the number of recipients in the email and try again.