Most error messages that an inbound sender will encounter here are discussed in our "Rejected Connections" article; others you may see that indicate a problem with the message being sent will include:
- 550 5.1.1 - Invalid mailbox: RECIPIENT@SOMETHING.RR.COM - This means that the recipient address specified in the error message does not exist here or is not an active account here.
- 552 5.3.4 - Message too large; message rejected. - You tried to send a message that is larger than we're willing to accept; we limit inbound messages to 30 MB.
- 554 Virus detected.
- 550 5.7.1 - Message rejected; message contains virus.
- 554 5.7.1 - Null sender with multiple recipients not allowed here - We do accept messages from the NULL sender (i.e., MAIL FROM: <>) but we limit such messages to one recipient.
The NULL sender is used for Delivery Status Notifications and similar messages; DSNs are sent in response to other messages, and since a message can only ever have one sender, a DSN should only have one recipient.
- 553 5.1.2 - Recipient address not local. relaying denied
The following error messages indicate a problem with our servers, not with the sending server or the message being sent:
- 421 4.3.2 - Connection refused. Server is busy, please try back later
- 451 4.3.0 - Internal error
- 451 4.3.2 - Internal error