Not all mail packages are compatible. There are a number of ways of encoding attached files. BinHex UUencode Make sure that who ever is sending you stuff is using a form of encoding that your mail reader can handle. UUencode is probably the best, but not all mail readers can handle it. You may need to get hold of extra software to UUdecode UUencoded attachments.
Chances are, either the email address was wrong, or the computer at the other end was busy.
Check you've got the email address absolutely right. Make sure there are no spaces or commas, as this will cause it to go wrong. If the email system can't find the address you typed, it will send it back to you. It may be that the other person's email system isn't working, or is full. If this happens, your message will be returned to you to let you know that it wasn't delivered.