You might see the below error messages while looking for incoming email rejects that shall help you troubleshoot your query. The below table shall help you understand the rejects well.
| Reply Code | Status Code | Error Message : Eg | Explanation |
| 550 | 5.5.1 | 550 5.5.1 Protocol error | Error message indicating that there is a problem with the way the email is being sent, usually related to the protocol commands or their sequence. |
| 550 | 5.1.8 | 550 5.1.8 <user@example.org>: Sender address rejected: Domain not found | Email rejected as no DNS MX and no DNS A record for the sender domain. These records specify the mail servers responsible for receiving emails on behalf of the sender domain. If an email cannot be delivered, a bounce message is sent back to the sender. Without proper MX and A records, the sender domain cannot receive these bounce messages, making it difficult to manage and rectify email delivery issues. |
| 550 | 5.7.27 | 550 5.7.27 <user@example.org>: Sender address rejected: Domain example.org does not accept mail (nullMX) | Email rejected as a malformed MX record such as a record with a zero-length MX hostname found for the sender domain |
| 550 | 5.1.1 | 550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table | Recipient email address does not exist |
| 550 | 5.1.1 | 550 5.1.1 <$UserEmailAddress>: Recipient address rejected: User unknown in local recipient table | Recipient email address does not exist |
| 554 | 5.7.1 | 554 5.7.1 <user@example.com>: Relay access denied; | Recipient email address does not exist |
| 552 | 5.2.2 | 552 5.2.2 user@example.com: Recipient address rejected: Quota exceeded (mailbox for user is full) | Titan enforces disk quotas to ensure that users do not exceed their allocated storage space |
| 554 | 5.7.1 | 554 5.7.1 Service unavailable; Client host [<ip>] blocked using b.barracudacentral.org; | IP address is found on an RBL, Titan rejected the email from that suspicious IP |
| 554 | 5.7.1 | 554 5.7.1 command rejected | IP address is suspicious, Titan rejected the email from that suspicious IP |
| 550 | 5.7.1 | 550 5.7.1 <user@example.com>: Recipient address rejected: Message rejected due to: SPF fail - not authorized. | SPF Hard failure : The SPF record for the domain specifies a set of IP addresses or domains that are allowed to send emails on behalf of that domain. If the IP address of the sending mail server is not listed in the SPF record, and the SPF record explicitly indicates that the IP address is not authorized (e.g., through the "-all" mechanism), the receiving mail server will consider the email as a hard failure. |
| 550 | 5.7.25 | 450 4.2.0 <user@example.com>: Recipient address rejected: greylisted : please retry after sometime; | When an email is received from a suspicious IP address (often due to issues with reverse DNS), the server temporarily defers the message. The sending mail server is expected to hold the email for a short period before attempting to resend it. Subsequent retries from the sending server will then be accepted. |
| 550 | 5.4.7 | 550 5.4.7 <user@example.com>: Recipient address rejected: Recipients Domain Hourly Quota Exceeded |
Ratelimit exceeded https://support.titan.email/hc/en-us/articles/360038836914-How-many-emails-can-I-send-and-receive?utm_campaign=Titan+Support&utm_source=support-email |
| 550 | 5.4.7 | 550 5.4.7 <user@example.com>: Recipient address rejected: Recipients Domain Daily Quota Exceeded |
Ratelimit exceeded https://support.titan.email/hc/en-us/articles/360038836914-How-many-emails-can-I-send-and-receive?utm_campaign=Titan+Support&utm_source=support-email |
| 550 | 5.4.6 | 550 5.4.6 <user@example.com>: Recipient address rejected: Recipient Quota Exceeded; |
Ratelimit exceeded https://support.titan.email/hc/en-us/articles/360038836914-How-many-emails-can-I-send-and-receive?utm_campaign=Titan+Support&utm_source=support-email |
| 550 | 5.4.6 | 550 5.4.6 <user@example.com>: Recipient address rejected: Mailbox Does Not Exist : Domains Hourly Quota Exceeded; |
Ratelimit exceeded https://support.titan.email/hc/en-us/articles/360038836914-How-many-emails-can-I-send-and-receive?utm_campaign=Titan+Support&utm_source=support-email |
| 550 | 5.4.6 | 550 5.4.6 <user@example.com>: Recipient address rejected: Mailbox Does Not Exist : Domains Daily Quota Exceeded; |
Ratelimit exceeded https://support.titan.email/hc/en-us/articles/360038836914-How-many-emails-can-I-send-and-receive?utm_campaign=Titan+Support&utm_source=support-email |
| 550 | 5.7.1 | 550 5.7.1 message content rejected |
Ratelimit exceeded https://support.titan.email/hc/en-us/articles/360038836914-How-many-emails-can-I-send-and-receive?utm_campaign=Titan+Support&utm_source=support-email |
| 550 | 5.2.1 | 550 5.2.1 <user@example.com>: Recipient address rejected: Incoming suspended due to abuse | This domain is suspended due to suspicious activity , currently incoming emails are not getting accepted |