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.

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.

550 5.3.2 Service currently unavailable

The error code 5.3.2 usually means that the system is not accepting network messages. We would thus need to look outside Exchange for a connectivity problem.

The error code ideally occurs ideally for two reasons which are listed below:

1. The sending IP is blacklisted at an RBL or blacklist monitoring site. (You may verify the same by inputting the IP via this link: http://mxtoolbox.com/blacklists.aspx )
2. There is a temporary block on the IP on our Inbound server due to multiple mails from this IP. Feel free to contact our support desk for more information and regarding de-listing the same.
3. If sending IP does not have a reverse PTR record configured. Reference URL: http://mxtoolbox.com/NetworkTools.aspx

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= 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.

554 5.7.1 : Relay access denied

There are couple of reasons for this error while sending an email
Please contact support.

551 5.7.1 The message was rejected due to classification as Virus,Spam or high bulk ratio

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.

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 4.1.8 : Sender address rejected: Domain not found

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.

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 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.