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Errors Sending Messages

550 SPF error received when trying to send a message

You may receive a 550 SPF error message if your domain is not configured correctly, specifically does not contain or contains an incorrectly set “SPF”...

Written by mbadmin
2025-12-28

Block specific attachments in emails

Email attachments are one of the most common ways malware is distributed. Blocking high-risk file types can help reduce the risk of malicious software reaching...

Written by mbadmin
2025-12-28

How does Greylisting work? Anti-spam protection

Greylisting is an anti-spam technique used by mail servers to reduce the number of unwanted messages reaching your inbox. It works by temporarily rejecting messages...

Written by mbadmin
2025-12-18

Possible reasons for message failure

In many cases, if your message cannot be delivered to the recipient, you will receive an automatic return email, which will include, among other things,...

Written by mbadmin
2025-12-19

SMTP Error Codes and Their Meaning

When an email cannot be delivered successfully, the sending mail server typically returns an SMTP error code. These codes help identify the reason for the...

Written by mbadmin
2025-12-19

Why didn’t I receive a feedback message about an undelivered email?

Typically, if an email cannot be delivered, the receiving server sends a Non-Delivery Report (NDR) or “bounce-back” message to the sender explaining why. If your...

Written by mbadmin
2025-12-22

Troubleshooting Outlook Errors 0x8004210B and 0x80042109

Errors 0x8004210B and 0x80042109 occur when your email client (such as Microsoft Outlook, Outlook Express, or Windows Live Mail) cannot establish a connection with the...

Written by mbadmin
2026-04-07

Troubleshooting Outlook Errors 0x800CCC0E and 0x800CCC19

Errors 0x800CCC0E and 0x800CCC19 typically indicate a communication failure between your email client (such as Outlook or Windows Live Mail) and the mybox mail servers....

Written by mbadmin
2026-04-07

Troubleshooting Outlook Error 0x800CCC18

Error 0x800CCC18 occurs when your email client (such as Microsoft Outlook, Outlook Express, or Windows Live Mail) attempts to log in using a specific authentication...

Written by mbadmin
2026-04-07

Troubleshooting Outlook Error 0x800CCC0E

Error 0x800CCC0E is a “Connection Failed” notification. It indicates that your email client (such as Outlook or Windows Live Mail) is unable to establish a...

Written by mbadmin
2026-04-07

Troubleshooting Outlook Error 0x8004210A

Error 0x8004210A is a “Timeout” notification. It occurs when your email client (such as Microsoft Outlook or Windows Live Mail) establishes a connection with the...

Written by mbadmin
2026-04-07

Troubleshooting Outlook Error 0x800C0133

Error 0x800C0133 is a data storage notification. It occurs when your email client (such as Microsoft Outlook, Outlook Express, or Windows Live Mail) is unable...

Written by mbadmin
2026-04-07

Troubleshooting Outlook Error 0x800CCC79

Error 0x800CCC79 is a delivery failure notification accompanied by the message “Relay access denied.” This occurs when your email client (such as Microsoft Outlook or...

Written by mbadmin
2026-04-07

Troubleshooting Outlook Error 0x800CCC15

Error 0x800CCC15 is a local data integrity notification. It occurs when your email client (such as Microsoft Outlook or Windows Live Mail) encounters a “Socket...

Written by mbadmin
2026-04-07

Troubleshooting the 550 SPF Error

Receiving a 550 SPF error (often accompanied by the message “Sender Policy Framework fail”) means the recipient’s mail server rejected your email because it could...

Written by mbadmin
2026-04-07

Troubleshooting email deliverability issues

Troubleshooting email deliverability issues Email deliverability depends on a series of security “handshakes” between your mybox server and the recipient’s inbox. If these handshakes fail,...

Written by mbadmin
2026-04-23

Sender address rejected: not logged in

This error appears when an email client tries to send a message without properly authenticating to the outgoing mail server. In most cases, it means...

Written by mbadmin
2026-06-02

Recipient address rejected: User unknown in local recipient table

This error is returned by the mail server when it cannot deliver a message because the recipient mailbox does not exist on the destination server....

Written by mbadmin
2026-06-02