Various SMTP error codes explained

550 5.1.1 Recipient address rejected: User unknown in virtual alias table;

This is the bounce back message that is received by the sender from our server if the destination email address does not exist.

The same error message is received if the recipient has a mail loop. That is, if the recipient has set a forward to another email address and that email address is forwarded back to the original one. In this case, you need to remove the loop in order to receive the emails.

If there is a forward set for an account, and an email is sent to that account, if for some reason the forwarded email is not sent, the original sender may get the above error.
For example, abc@domain.com if forwarded to abc@dom.com, if a third user xyz@dom2.com emails to abc@domain.com, he may get the above error if the forwarded email is bounced.

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.

550 5.7.1 : Helo command rejected: You aren't localhost.
550 5.7.1 : Helo command rejected: You aren't localhost.localdomain.
550 5.7.1 : Helo command rejected: You are not me
Our servers do not accept SMTP HELO command as HELO localhost or HELO localhost.localdomain or HELO . We accept HELO from a valid Domain Name or your computer name which is other than your domain name. Please check with your ISP or Mail administrator for this issue.

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.

554 5.7.1 : Relay access denied;
There are couple of reasons for this error while sending an email

If the domain is in the status "Pending Verification" in the control panel.
If the user has not checked the option "Our server requires authentication" option while setting up the account. For more information on setting up the email account, please refer: http://support.mailhostbox.com/email-users-guide/configure-email-client

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.

530 5.7.0 Recipient address rejected: Authentication Required.
This error comes when your sending mails to remote domain without using smtp authentication.

550 5.4.5 Recipient address rejected: Hourly domain sending quota exceeded
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.

550 5.4.6 Recipient address rejected: Hourly sending quota exceeded
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.

553 5.7.1 Sender address rejected: not owned by user user@domain.com
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,

Log into the webmail of user1@domain.com
Add the identity from the settings tab for user@domain.com (If the from address is user@domain.com). For more information, please refer: http://support.mailhostbox.com/email-users-guide/sender-identities
Once the identity is added and confirmed, you should be able to send the emails.

554 5.7.1 : Recipient address rejected: USER IS SUSPENDED
This error comes if user is suspended in Control Panel.

522 5.7.1 : Recipient address rejected: Requested mail action aborted: exceeded storage allocation
This error comes if the user is overquota.

554 5.7.1 : Sender address rejected: Access denied.
This error comes when the sender address is blocked on our server. Generally this is done if spam has originated from this user.

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.

450 4.7.1 : Recipient address rejected: Policy Rejection- Quota Exceeded
This error comes from our incoming mail server if user is receiving excessive amounts of mails.

450 4.7.1 Recipient address rejected: Access denied.
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 15 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.