If the error received was blocked using relays.ordb.org, reason: Blackholed by ORDB , then the message was rejected because the sender was using an insecure email system. The email system they used is what is known as an "open relay", which means that no security of any kind has been implemented for accessing the server. That means that "spammers" (people that send millions of unsolicited advertising messages) can log on to that server and use it for distributing messages. Most commercial email sites, including ours, block messages from insecure servers because 99.9% of messages originating from them are spam, which is wasteful on resources and annoying for users.
Ask the sender to pass on this information to their system administrator or ISP, including the following link: http://ordb.org/lookup/?host=xxx.xxx.xxx.xxx (where 'xxx' is replaced with the information in the original error 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 possibly they have been blocked by the mail abuse systems described in Email Resource Limits .
The server blocks a few emails with certain attachments due to security reasons. The blocked attachments are:
double-extension files (eg something.doc.exe): Many Windows systems hide all but the first file extension by default, so a file called 'something.txt.exe' will appear as 'something.txt', apparently a harmless text file when it's really an executable program. This is a common trick viruses use to try and spread themselves.
all .pif, .scr, .cpl and .com files: There is little reason to be sending these files, and they are commonly used by viruses as a way to spread. If you need to send one of these files, zip it first, then send the zip file.
files named any of the names "details", "encrypted", "first_part", "gift", "information", "moreinfo", "pub_document", "text_document", "TextDocument", "body", "text", "document", "message", "data", "file", "readme", "doc" and with the extensions .bat, .exe, .cmd, .zip or .rar: Common names that were used by Novarg virus, but still appear to be going around.
The server has a feature called "duplicate delivery suppression". Basically, most messages have a unique identifier called the "message id". For each message that arrives for an account, the message id is noted. If a second message with the same id arrives, then it is suppressed. This is used to stop infinite mail loops overloading your account or our system. However, it can result in odd behaviour where when the same message arrives from different sources, only the first one will actually be delivered, the rest suppressed.