Attachment names may only contain one "." and may not contain multiple spaces in a row. For example, "filename.txt.doc" is not allowed nor is "filename txt.doc" (there are 2 spaces after the "filename". If you must send a file that contains this type of naming you will need to either rename the file or ZIP the file and send the ZIPPED file to the recipient.
To send a file that ends in one of the following extensions you need to either rename the file extension or ZIP the file and send the ZIPPED file to the recipient. Please note, however, that you may not encrypt the .zip file or any other archive before sending or our mail servers will deny access to the encrypted archive. The reason for this is several viruses recently have included encrypted archives as attachments and it's difficult to scan an encrypted archive for viruses.