I'm getting a 501 Syntax Error when I try to e-mail someone. They can send me mail. Check that your mail goes out using your proper e-mail address. Someone needs to send me a large file. E-mail is not necessarily an appropriate means for transferring large files.
Many systems limit the size of incoming messages.
If a message is being rejected because it's too large, explore an alternate means, such as ftp. The arrival date indicates when the message arrived in your mailbox. It's determined by your mail server. With these points in mind, then, you should be able to determine the source of your problem. To combat junk e-mail, mail servers throughout the Internet are being configured to reject mail if the sender's host does not appear to exist.