Someone tried to send me a message, but it bounced back

If the error received was "blocked using" or similar such message, then the message was rejected because the sender was using a sending email system that's been detected as compromised in some way. Most commercial email sites, including ours, block messages from insecure servers because almost all messages originating from them are spam.

Ask the sender to pass on this information to their system administrator or ISP, including any other links or information in the block message.

That link is to the sender's email server's entry in a database of servers known to be insecure and sources of spam. Because their email provider is on that list, their messages to hundreds of thousands of destinations will be bounced or silently ignored.

If the error received was content rejected, then the message was rejected because it either contained a probable virus, or it is a message previously reported as being spam. If a message is flagged as a possible virus incorrectly, ask the sender to zip it before sending.

If the error received was access denied, the sender or recipient has been blocked by the server. Possibly they have been reported as a spammer, or they may have been blocked by our mail abuse systems.

An email with an attachment bounced back

We reject emails with certain attachments due to security reasons. The blocked attachments are:

The same email from multiple sources fails to arrive

Most messages have a unique identifier called the "message id". When a message arrives at an account, the message id is noted. If a second message with the same id arrives, it is ignored and silently discarded. This is used to stop infinite mail loops overloading your account or our system. However, it can result in odd behaviour when the same message arrives from different sources, as only the first one will actually be delivered.